They're beating the bongo drums again for a resumption of vacation travel by U.S. citizens to Cuba, but it's not music to the Treasury Department. And there's a hint of a Mexican connection.

The growing cast of characters in the "Curious Case of the Cuban Caper" includes the State Department; Treasury; Cubatur, the Cuban government tourist agency; Unitours, a Canadian tour operator, the American Society of Travel Agents (ASTA); and Mexicana Airlines. The strange scenario appears to revolve around high-level diplomatic maneuvers, the need of the Cuban government for U.S. dollars to bolster its sagging economy, and the eagerness of some travel industry officials in Mexico and the United States to get a piece of the action.

There are no diplomatic relations between the United States and Cuba, and a U.S. travel "ban" remains in effect.

Among the latest developments:

The Secretary of State will "routinely review" passport restriction policies as they apply to four countries, including Cuba, next month. "The review occurs every six months and I don't think anyone can speculate on the outcome," a spokesman said. (The other countries are North Korea, Vietnam and Cambodia.)

Even if the Cuban travel restrictions were lifted, a trade embargo would remain in effect unless removed or modified by a possible combination of executive and congressional action, according to informed sources.

The question of whether Americans will be permitted to travel to Cuba for pleasure in groups via Mexico is expected to be raised by President Lopez Portillo in his meeting this week with President Carter, according to a sales official of Mexicana Airlines. (Mexicana normally does not engage in flights of diplomacy and there is no corroboration in Mexico or Washington.) No agenda for the White House talks has been released.

Eduardo Ferrer, U.S. sales manager based at Mexicana's office in Burlingame, Calif., said a related report in an American trade publication about plans for "familiarization" flights via Mexico for u.S. travel agents "shouldn't have appeared . . . we have instructions from Mexico that we should do nothing in this area until we have a clear understanding" (as a result of the White House meeting). The trade story made no mention of the upcoming talks.

The "fam trips," designed to brief agents and wholesalers so they can prepare and market group packaged tours for their American clients, are supposed to be flown free to Havana by Mexicana in connection with Cuba-Mex, a Mexico City-based travel agency, Ferrer confirmed in a telphone interview. Agents would be guests of the Cuban government.

CubaMex, which specializes in travel to Cuba, hopes to act later as the tour operator for U.S. groups if the travel ban is eased, manager Fousto Franko said last week in a telephone interview. Meanwhile, he said that his plans were "firm" to fly a group of U.S. travel agents to Mexico and then on to Havana for four days, aboard a regular Mexicana flight, beginning March 3.

Cuba, though eager to welcome U.S. travelers, is not encouraging visits by individual tourists now because it feels it can better handle groups with its limited resources.

Alex Lopez, an Arlington-based travel coordinator, returned here on Feb. 1 with 17 "wholesalers, agents and coordinators" from what he said would be his "first and only" familiarization trip to Cuba until U.S. restrictions are lifted. They flew via Air Jamaica from Kingston. Their host, Cubatur, continues to issue invitations to U.S. travel agents despite questions raised by the Treasury Department, Lopez said. The Cubans "are counting" on a lifting of the travel ban, he said.

The American Society of Travel Agents was preparing last week to notify its approximately 10,000 members "to observe thelaws of the land, and if the laws of the land say they should not go to Cuba they should not go," a spokesman said in New York.

Stanley Sommerfield, acting director of the Treasury Department's Office of Foreign Assets Control, issued a sharp warning recently concerning plans by a Toronto-based tour wholesaler to fly U.S. tourists from Canada to Havana on a seven-day winter package. Unitours has run newspaper advertisements in New York, Boston, Chicago and Seattle.

Unitours, in a statement issued Feb. 4, one day after being informed by The Washington Post of Sommerfield's warning during a telphone interview, announced it was suspending "for the time being its tours and would also "suspend promotional activities temporarily in the United States."

In the Feb. 3 interview, George whitfield, vice president of marketing for Unitours, had said that his firm was still accepting bookings and visa applications and had had "no official correspondence from Sommerfield or his department." A number of visa applications had already been submitted to the Cuban Consulate in Toronto for approval.

Sommerfield had called the Unitours trips "completely illegal" for U.S. citizens and "a blatant attempt to induce Americans to violate the law." He said that Americans going on the tours could be found guilty of "violating the Trading With the Enemy Act of 1917," and termed the Unitours efforts "reprehensible . . . we will do our very utmost to suppress it." Some "actions" have already been taken, he added, but declined to elaborate.

Sandy Cameron, vice president of operations, said last week that his firm realized that "the Treasury Department would be in a postion to put administrative controls on bank accounts" in connection with tour payments made in dollars by U.S. citizens. "Payments would go from our Toronto bank to U.S. banks for clearance," Cameron said.

"Unitours has not mounted a concerted marketing effort in the United States," Whitfield had explained. "We have merely promoted in four selected cities . . . to test levels of response and official reaction. I can't regard 480 requests and no more than two dozen actual visa applications received as being indicative of any mass movement."

Whitfield said his firm's seven-day Cuban Holiday programs range in cost from $375 to $475, plus $8 Cuban transportation tax, and include air fare, hotels, meals and all expenses except for drinks, souvenirs and haircuts. There is no-tipping in Cuba.

Graham Atkin, Unitours president, said in his statement that there are "strong indications that the new U.S. administration may, in the near future, relax its attitude toward Cuba . . ."

Since the early 1960s American passports have contained a notation that they are not valid for travel to Cuba, though validation is occasionally given in special noncommercial case - such as for members of Congress, medical people, journalists and for humanitarian reasons.

Individual American travelers can still elect to visit Cuba without a passport and the protection of their government, the State Department has acknowledged, and there have been no attempts to prosecute anyone for strictly pleasure travel. But the Treasury Department has pointed out that technically even the act of "cashing travelers' checks" is a type of "commercial transaction."

Sommerfield has indicated that primary attention, with a view toward prosecution, was being directed now at commercial operators. But he added that, "certainly, if there were tourists reserving space into Cuban hotels through a commercial agency in Canada, Mexico or elsewhere, we would apply the same administrative measures to the agencies."

Sommerfield refused to speculate on possible easing of travel restrictions. "I am enforcing the law as it now exists." But he added that "presumably" the licensing procedure, which is part of the embargo, would have to change if the travel ban is modified.

His authority stems from the Cuban Assets Control Regulations, which forbid U.S. citizens to spend money in Cuba without a license from the Treasury Department. Such licenses are granted only to travelers whose passports have been validated by the State Department for noncommercial Cuban travel.

Americans responding to Unitours ads were told that visas would be stamped on a separate piece of paper, not in their passports.

According to a series of court decisions, including a 1967 Supreme Court ruling, U.S. citizens not using a passport cannot be prosecuted for pleasure travel to Cuba or any country deemed off-limits by the State Department. Though a citizen normally requires a passport to enter or leave the United States, and the Secretary of State does have the authority to impose restrictions on the use of the passport, the courts have ruled that he does not have the authority to impose restrictions on an individual's travel.

But Sommerfield has emphasized that any expenditure of money in Cuba by unlicensed U.S. visitors is illegal. He has stated that it would be a "criminal offense" if a U.S. tour operator or travel agent visiting the island became involved in a commercial venture to send tourist to Cuba, with money being transmitted directly or indirectly.

Agents who go on "fam" trips presumably would not be spending funds on the trip since expenses are paid by the Cuban tourist agency.

Mexicana's Ferrer said that his airline has "eight gateway cities" from the U.S. to Havana via Mexico City. If U.S. restrictions were modified, he said, Mexicana would have to apply to the U.S. civil Aeronautics Board (CAB) for authority to fly U.S. agents free (a standard procedure), since the trip would actually originate when an agent boarded a flight in this country. Tourists would still pay.

Such a modification, which would probably include Canada as well as Mexico if it became a reality, would not necessarily mean there would be early consideration by the Carter administration of direct U.S.-Havana flights.

There could be "no bilateral air agreements without diplomatic relations," Alex Lopez, 27, of Global Travel in Arlington, observed, "but charters can be arranged." Unitours now uses regular scheduled airlines (Air Canada and Cubana) for its weekly tour departures from Toronto and Montreal, and flights can be added. In addition, there would be the Mexican connection.

The State Department has stated in the past that "a dialogue" between the United States and Cuba will not be resumed until all organized Cuban military units have been removed from Angola. Cuban "agitation" for Puerto Rican independence also affected earlier moves to ease tensions between the two countries.

And on Feb. 2, the State Department took issue with U.N. Ambassador Andrew Young's contention that Cuban troops "bring a certain stability and order" to Angola.

Yet Secretary of State Cyrus R. Vance has also said that he was not setting "any pre-conditions at this point" regarding normalizing relations. Thus the administration may be tourism as a means of reopening the dialogue with FidelCastro.

Some U.S. tour operators have been eager to cash in on what many travel experts believe could grow into a booming business. Havana was known in palmier days as the "Pearl of the Antilles." Peak year for Cuban tourism was 1957, when 272,000 visited the island. In 1975, there were nearly 40,000 foreign visitors and Cuban projected 80,000 for last year. Canadians on group tours now make up more than 50 per cent of Cuba's visitors.

Born in Cuba, Lopez is a naturalized American who, though eager to lead tours to his homeland, does not want to break any laws. He said he agreed to assist Cubatur in handling the "fam trip" just concluded only after an earlier trip involving an out-of-town associate ran into organizing problems. Lopez said the Cubans are "very much interested" in an exchange of professionals such as doctors, teachers and researchers.

Though the Cubans are quite willing to arrange interviews with factory workers, show off what they believe are teh benefits of the revolution, and also permit sidetrips to the scene of the ill-fated Bay of Pigs invasion area, they are not interested in regimenting tourist activity, Lopez maintains. "You can come and lie down on the bench for a week if you wish, they don't mind," he said.