Assistant Secretary of State Langhorne A. Motley is 46, not 47 as reported yesterday.

In the whirl of changing faces and rumors of change in the State Department, particularly in the Central America division, Langhorne A. Motley is conspicuous by the solidity with which he appears entrenched as assistant secretary of state for inter-American affairs.

That fact has surprised a lot of people. Motley, 47, is a maverick among diplomats, a wisecracking, fast-talking, aggressive fellow whom many on Capitol Hill once dismissed as a lightweight. But he is scheduled today to make what undoubtedly will be the first of many appearances before the 99th Congress as the Reagan administration's leading defender of its controversial policy in Central America.

Members of the House Foreign Affairs subcommittee on the Western Hemisphere say they know what to expect.

"Every time he comes up here with all his charts and graphs, I have the feeling he's selling us condominiums," said one committee staff member.

"He's a cowboy, he likes to shock a little," said one outspoken Democrat who opposes most of what Motley is pushing. "But he's very effective. It's hard to get a handle on him."

Motley, who insists on being called Tony, was a real estate magnate in Alaska and likes to sport cowboy boots with the 20-year-old Air Force jacket that still fits his compact, energetic frame. In 18 months as assistant secretary, he has been called a rake, a rascal, superficial and worse for breezily ignoring his prepared texts in public, sliding over details and confiding dubious jokes and unprintable opinions in private.

"I do spend whole days operating on the seat of my pants," he admitted in a recent interview, "but I like to think of that as a quality of instinctive judgment. I like to think one of my fortes is instinctive judgment."

Even Motley's critics say he has returned power over Central American policy to the State Department and has become one of the administration's better fence-menders on Capitol Hill.

"No, I wouldn't call him a survivor -- he's a winner," said a veteran State Department Latin hand.

The best Motley anecdotes show a taste for brink-walking that few diplomats would risk. He used to tell a joke that said El Salvador was like Costa Rica because neither had an army. While this was true in Costa Rica, it was heresy for an American official to say about El Salvador, where the administration was and is providing massive military aid to stave off a leftist rebellion.

He was asked to comment on a British diplomat's observation that Soviets as chess players were better at handling stalemate than Americans who play poker. "Having lived in England for three years," he said, "I've found that Englishmen, aside from having weak chins, are marvelous guys at one-liners."

Sen. Christopher J. Dodd (D-Conn.), a firm critic of Reagan's policies, said Motley once sent him a poster and a flyer from El Salvador's right-wing ARENA party that showed a vulture attacking two rats, one labeled Dodd and the other labeled for another critic, then-Rep. Clarence D. Long (D-Md.).

Dodd was not insulted. "I personally like the guy," Dodd said. "He's much more fun to deal with than Enders . . . . You could dislike Enders personally, which made it easier to attack his policies."

The brooding, intellectual Thomas O. Enders preceded Motley as assistant secretary and is now ambassador to Spain. Motley went the other way on the career ladder: He was ambassador to Brazil when summoned to Washington and came somewhat reluctantly.

He was born in Rio de Janeiro of American parents and lived there as a self-described "Carioca street kid" until he was 17. He spent 10 years in the Air Force and then amassed a fortune in Alaska, learning the Washington ropes as a fervent Republican while lobbying for four years in favor of the 1980 Alaska lands bill. "I've been had by pros," he said.

Being U.S. ambassador to Brazil, he said, was "an emotional ambition. . . something to cap off my life." Now, however, he said, "I don't consider my life capped off." At the same time, he said he has no plans to leave his current post.

Motley arrived on the job in July 1983, just as the United States was increasing its involvement in Central America with stepped-up military exercises and as the presidential advisory commission on the region, headed by former secretary of state Henry A. Kissinger, began its work. After Enders' departure, Central America policy control had moved to the White House, but Motley has worked closely with Secretary of State George P. Shultz to bring it back to Foggy Bottom.

In his 18 months, Motley has presided over -- and by all accounts pushed hard for -- a major expansion of U.S. aid to Central America, the invasion of Grenada, moves to curb death squad activity in El Salvador, and both the opening of direct negotiations with Nicaragua and a toughening of the U.S. line toward Nicaragua's leftist Sandinista government.

The "contra" rebels against the Sandinistas have grown from an estimated 8,000 irregulars in mid-1983 to about 15,000 now with help from the Central Intelligence Agency, and Motley gives no sign of any loss of faith in their effectiveness, despite recurrent serious flaps with Congress over their tactics.

In fact, he refuses to discuss any differences he might have with official policy or how those differences are worked out in private. Asked whether he feels pressed, he joked: "Most of the time I have two options: either the unattainable or the unacceptable."

"What I think about it doesn't amount to a hill of beans," he said, "but I don't think I'm considered in meetings to be a shrinking violet." He said the White House often asks him "to carry water" on Capitol Hill on issues unrelated to Central America because of his contacts there.

A key subordinate said Motley is "ferociously punctual" and shares with Shultz a businessman's traits of delegating authority and demanding loyalty and success. Another said: "His meetings are like a football huddle: 'Whaddaya got? Whaddaya got? Let's go on that.' He'll cut you short and interrupt, but nobody takes offense because you can do the same to him." At least four loyal subordinates are scheduled to become first-time ambassadors in the current reshuffling.

But even Motley's friends describe him as "a real street fighter" when crossed. He cut former special ambassador Richard Stone out of key decisions and finally forced his resignation after Stone was imposed on him by Rep. Long.

"What's important is that what the president and the secretary have decided is carried out," Motley said. "My loyalty runs to them, that's all; it's very simple, there's no doubt in my mind how it works."