When Datsun chief Takashi Ishihara came to the United States two years ago to explore investment opportunities for the giant auto firm, the man who took him around Capitol Hill to meet such notables as Senate Republican Leader Howard H. Baker Jr. (Tenn.) and House Speaker Thomas P. O'Neill (D-Mass.) was Richard V. Allen, a business consultant to Datsun.

That visit, commemorated in a photo of Allen, Baker, Ishihara and other smiling Datsun officials, underscores an important point about Allen, the 44-year-old former Nixon administration official who is the key foreign-affairs adviser to Ronald Reagan and a prime possibilty to become the president's national security affairs adviser in a Regan administration.

In recent administrations, security affairs advisers and secretaries of state have tended to be scholars of high reputation like Henry A. Kissinger and Zbigniew Brezezinski or skilled negotiators like superlawyers Cyrus R. Vance and William Rogers.

Allen, however, has a different background. He began as a academic, wrote or edited (sometimes with others) five books on communism (none in the last 10 years).He worked on strategic problems at the Hoover Institution and the Georgetown Center for Strategic and International Studies. He served at the National Security Council and then as a key White House aide on trade matters during two separate stints in the Nixon administration.

But his reputation as a scholar doesn't begin to match that of Kissinger or Brzezinski, and he never received a Ph.D.

In between his White House jobs and for the eight years, he's moved out of the scholarly world to a much different one: that of the international business consultant, criss-crossing Atlantic and Pacific to consult with Japanese or Portuguese clients, to look, into business opportunities in Taiwan, to explore investment policies in Angola and Mozambique. His firm's name here is Potomac International Corp.

Allen declined to volunteer the names of any of his clients, but The Washington Post learned (and he later confirmed though not in every detail) that he is a consultant to Datsun (Nisan-U.S.A.), although he disputes reports that he gets $100,000 a year. He was once a consultant and registered foreign agent at $60,000 a year to a group of Portuguese companies with investment interests in Angola and Mozambique. He was a consultant at $4,000 a year from 1974 to 1977 to Tokyo Electric Power, Japan's largest utility. He is a consultant to the Industrial Research Institute of Japan, an energy thinktank, at about $2,200 a year; and an occasional consultant to Lockheed on plane sales prospects ($4,800 in 1976; $1,000 in 1978).

Allen also said, again declining to provide names, that he has "a range of domestic clients in this country, some interested in energy, some transportation, some government contracts."

What Allen provides clients is a combination of knowledge about international security and national policy issues and about trade, gleaned from academic and White House experience. And he is articulate and fast on his feet.

Allen's work has put him in contact with some interesting and notorious people. Between White House jobs, he worked in 1970 for the King business empire, a vast conglomerate with oil and gas holdings, drilling funds and tax shelters. The head of the conglomerate, John King, was later convicted of stock fraud, but no assertions of wrongdoing were ever made about Allen, according to the Securities and Exchange Commission.

In 1972, Allen did consulting work at $10,000 a month for Howard Cerny, a lawyer representing Robert Vesco. Vesco was later accused of bilking investors of hundreds of millions of dollars. Allen was studying possible creation of an international financial center in which Vesco was interested. Allen was never accused of involvement in Vesco's alleged swindling, but in July, when the magazine Mother Jones ran an article on Allen's consulting for Cerny and meetings with Vesco, questions were raised by some in the Reagan camp about the propriety of Allen's having associated himself with someone like Vesco. That concern appeared to have faded.

In 1976 Dr. Thomas Cheatham, a former official of Grumman International, testified to a Senate Foreign Relations subcommittee that in 1972 Allen had, in effect, suggested a $1 million Grumman contribution to President Nixon's reelection committee -- in return for White House help in pressing Japan to buy Grumman's E2C airborne warning plane. Cheatham didn't say that any money was ever paid, simply that Allen had suggested it when they met at the White House.

He said Allen had told him "people that he and the White House made an effort for should be grateful [and] in these terms, contributions to the Fund for the Reelection of the President would be very much in order." When Cheatham asked what he had in mind, he testified, Allen responded that "the E2 was worth $1 million."

Allen denied the charge under oath: "The allegation is false."

Company officials subsequently testified that after the alleged conversation about the contribution, Allen visited Grumman headquarters in Bethpage, N.Y., was briefed on the plane and also inspected an F14 fighter. But the officials said they and Grumman head Clinton Towl recalled nothing of any proposed $1 million contribution, with Towl reportedly asserting "I sure as hell would have," if it had been discussed with him.

Federal prosecutors were asked to look into the matter to determine who was lying, according to a Senate committee counsel, but reported back that they couldn't resolve the issue and never prosecuted anyone.

Cheatham, in a recent telephone interview (he is now a consultant in Southeast Asia), asserted, "I still affirm the story."

Emmett Anglin, one-time Grumman representative in Japan, said in a phone interview a few days ago that when Cheatham visited him in Tokoyo about four years before his Senate testimony, Cheatham told the same story that he later told the Senate committee.

Allen again angrily denied the Cheatham charge in an interview last week.

In Washington, where the daily bread of politics is rumor, unsubstantiated stories have clustered about Allen.

In the interests of clarification, Allen in an interview commented on a number of such rumors. He denied each one as "preposterous," "a lie" or "categorically untrue" and gave these specific responses:

"No, I did not" encourage and "absolutely did not indicate that Nixon approved" covert signals to the South Vietnamese to go slow in 1968 peace negotiations, lest a peace agreement help the Democrats beat candidate Nixon in the presidential election.

"I do not work for the CIA -- period! Never."

He hasn't any special link to Taiwan, but he has explored business possibilities there and visited there.

A story that he was somehow involved when Israelis made off with five French gunboats on Chirstmas Day 1969 is "preposterous."

In the Nixon White House, he was asked to head up a unit that later became known as 'the Plumbers." But at the time the job involved nothing more than declassification of historical documents, the unit wasn't called the Plumbers, he turned down the job, and he wasn't involved in any way in its later activities.

Allen was born Jan 1, 1936, in Collingswood, N.J. He received B.A. and M.A. degrees from Notre Dame in 1957 and 1958, had study grants of a few thousand dollars a year from the Helm Foundation from 1958 to 1960, and later worked toward a doctorate at the universities of Freiburg and Munich in West Germany.

In 1961-62 he was an assistant professor at Georgia Tech. From 1962 to 1966 he was a founder of and senior staffer at Georgetown's Center for Strategic and International Studies, and from 1966 to 1968 was at the Hoover Institution on War, Revolution and Peace at Stanford University. Both institutions are considered centers of hard-line anticommunist, anti-detente thought.

Allen is a Roman Catholic.He has seven children, whose picture he displays on the wall, not far from an indoor sandbox, which seashells in it, in which, it was reported when he was in the White House, he wiggled his toes to ease tension.

Once an avid private pilot, he stopped after breaking both legs and nearly getting killed in a Vermont accident in February 1973. He has property in Sanibel, Fla., a famous shell beach, a longtime home in Brant Beach, N.J., and a home here.

In 1967, Allen joined the Nixon-Agnew presidential campaign as an adviser and became director of foreign policy research. In December 1968, Nixon named him to the senior staff of the NSC.

After leaving the NSC in October 1969, he went to work in November International Resources Ltd., the King international oil and mineral affiliate.

Allen left King in mid-1971 and went back to the White House as deputy assistant to the president for international trade and economic policy, resigning in July 1972 to form Potomac International.

During this period, Allen is said to have correctly predicted that the Soviets would invade Czechoslovakia to squelch the moderate regime of Alexander Dubcek.

Many expected Allen to be named as Nixon's national security adviser in 1969. When Nixon instead named Kissinger, Allen became Kissinger's subordinate. Sources say Kissinger viewed Allen as too hardline, too anti-detente, and successfully boxed Allen out of any real decision making at the NSC. "They gave him the best office they could find but no in-box," said another former NSC official.

In those days, and toward the end of his Hoover period, Allen was usually called Dr. Allen.

It turned out, however, that he'd never received his doctorate. He said in a recent interview that he finished the dissertation at Munich, had it approved by all his major professors and received the top mark on his orals.

Other professionals, however, had dedmanded changes "on what I regard as political grounds. I refused."

At the about the time he was leaving the White House in mid-1972, Gil Straub, who turned out to be a Vesco associate, contacted him. They held discussions on international business conditions and he met with Vesco, even traveling on Vesco's plane. This was about the time he was forming Potomac International and going into business.

Later, Howard Cerny, who turned out to be Vesco's lawyer, and whom Allen had met at Vesco's, asked him to do some consulting work on international business conditions, exploring the possiblity of creating an international financial center, perhaps in the Caribbean.

Allen got $10,000 a month for about six months. He said he doesn't recall whether he knew immediately that Cerny's client for that particular study was Versco, but he soon came to know it: "I became aware Vesco was his client; at the time of hiring he didn't tell me Vesco specifically; he had other clients." Later he thought again and added, "I guess I knew Vesco was his client."

Cerny is less uncertain. He said flatly in a telephone interview that Allen certainly knew that the study was to be done for Vesco, and in fact, when he met Allen at Vesco's he assumed he worked for Vesco in some connection. But he added that the $10,000-a-month study was a straight, legitimate arms-length study.

Allen took Cerny to see William Casey, then SEC chairman, apparently on a social visit, and Cerny brought up Vesco's problems in an attempt to get the SEC to soften its stand. But Allen and Cerny say Allen hadn't any foreknowledge that Cerny would bring up this subject. Cerny claims he didn't intend to raise it, just blurted it out. Allen stalked out of the room and later apologized to Casey.

When the Vesco story was reported recently by Mother Jones magazine, the White House personnel office told reporters that, after leaving the White House, Allen had signed a contract from time to time during the period Aug. 9, 1972, to July 1, 1973. He was to work as a White House consultant -- be paid $150 a day, but only on days when actualy doing consultancy work. The White House thus led reporters to believe he'd been a paid White House consultant when he worked for Cerny.

However, Allen vigorously denied ever actually performing consultancy work for the White House during this period -- and the White House, Office of Personnel Management and National Archives recently conceded that in fact they don't know. They haven't any records to show whether he actually worked and was paid as a consultant; they say they can't find any.

An early Allen client (1973-74), at $60,000 a year plus expenses, was the Overseas Companies of Portugal (OCP).

According to Gerald Bender, associate professor and Portugal specialist at the University of Southern California, "It was well known that the Overseas Companies of Portugal were largely a front for the colonialist government of Portugal in an attempt to influence public opinion in the west in favor of continued Portuguese control of colonies in Africa."

Allen, however, in an interview and in 1973-74 filings with the Justice Department, indicated that his contract involved a straightforward relationship.

He said the 70-odd companies in OCP were anxious to persuade people that putting money into Angola and Mozambique wasn't too risky.

Allen, according to his filing with the Justice Deparment under the Foreign Agents Registration Act, agreed to engage in "political propaganda" by carrying on "private conversations" with public officials, legislators, editors and government agencies.

He helped arrange a trip by Rep. Philip Crane (R-Ill.) and his wife to Portugal. Angola and Mozambique (Allen's younger brother David went to work for Crane last May).

One Allen concern was the impact of press reports about the alleged "Wiriyamu" massacre in Mozambique in December 1972, in which government forces were said to have slaughtered 400 villagers suspected of antigovernment links.

Allen said in an interview that he believes the alleged masssacre "never did occur" and it appears to have been a Czech disinformation report."

Whether the massacre actually did occur as initially described was challenged by the Portuguese at the time and, according to press reports, was never absolutely established. A U.N. commission in 1974 concluded that there had been widespread atrocities by Portuguese troops in Mozambique durng that general period.

Asked why he registered as an agent for only one foreign client, Allen said he is merely a consultant, not one who takes material action on behalf of a client. "I'm not a lobbyist, not trying to change regulations or laws," and therefore isn't required to register. He said he'd discussed the matter with the Justice Department officials, and that Joel Lisker, who heads the registration unit, had told him he doesn't fall within the requirements for registration.

Many of Allen's friends believe that the Mother Jones article publicizing his Vesco connection, plus the dropping of hints in some circles that Allen's Portuguese, Azorean and other business links are somehow illicit, are simply smear attempts by people who dislike his views, who consider him too conservative.

"It's just an ideological attack, an attempt to twist dark meaning into perfectly aboveboard business relationships," said one Democrat.