The well-fed coffee planters and bespectacled businessmen lunching at the outdoor restaurant hardly bothered to look up as a lone, UH1H Huey helicopter came throbbing over their thatch-umbrellaed tables.

Not even the two-man band, the Machos, wheezing golden oldies with a Latin beat while well-dressed, lunching women gossiped and teen-agers swam in the adjoining pool, was fazed by the hollow thumping competition from the helicopter's rotors.

It was as if just another tropical insect were flitting by.

But to a visitor remembering other cities, other wars, it was an all-too-familiar scene: the affluent and the comfortable briefly losing themselves in forced gaiety to try to seal off the horrors that lurk just beyond the hotel patio's high protective walls.

To be sure, this is not the country-clubbish Cercle Sportif of Saigon, where th escapism was at once livelier, more abandoned and unreal. Nor is the atmosphere in the coldly modern Hotel Camino Real here anything like the seedy funkines of the old Hotel Royale in Phnom Penh.

The lone U.S.-made helicopter churning its way back to San Salvador's Ilopango airfield on the city's edge summons up only the ghost of a memory of the angry swarms of Hueys that thrashed overhead on their way home to Ta Son Nhut airbase from their raids in the Mekong Delta.

But the Huey in the sky, ignored as it was by the lunchtime gentry, was an inescapable sign of the U.S. stamp that is beginning to emerge on this dismal and ugly little Central American war.

Compared to Vietnam and Cambodia, those last tropical wars on which the United States left its imprint, the U.S. involvement in El Salvador is still small potatoes: some $35 million in military aid, 10 Hueys, thousands of M16 assault rifles and M79 grenade launchers, tons of ammunition, radios, trucks and 54 U.S. military trainers to show El Salvador's 18,000-man armed forces what to do with them.

Limited as the commitment here still is, it cannot escape some comparisons in American public opinion with those past wars -- even if the comparisons grate on an administration determined to purge the public conscience of past disasters.

Like it or not, the U.S. imprint on this struggle between leftist guerrillas, many of them communists, and the civilian-military junta that rules the cities, is growing by the day.

There are, of course, the Hueys which have replaced the few rickety French-made Allouettes previously flown by Salvador's small Air Force. And the new M16s are increasingly appearing in the hands of patrols in the bush to replace the heavier, West German-made G3s long used by Salvadoran infantrymen.

At the general staff headquarters, local noncoms are busily building large-scale, plaster-of-paris mockups of the country's rugged volcanic terrain so the 10-man U.S. tema helping to raise the general staff's planning and operations consciousness can use their standard-issue telescopic pointers to demonstrate the fine points of antiguerilla tactics.

These signs of U.S. influence are cropping up in spite of all efforts to keep the U.S. military profile in El Salvador studiously low, if not underground. Everything that could be done to sanitize the U.S. presence has been done.

Members of the U.S. military teams here insist on being called trainers to differentiate their rigidly prescribed functions of martial pedagogy from that of the Vietnam advisers who were allowed, indeed under orders, to accompany their charges on combat operations.

The 15 U.S. Special Forces officers from the U.S. Army School of the Americas in Panama are not supposed to be called Green Berets because -- as one U.S. official here insists -- they were under orders not to bring their traditional berets in their duffles.

For reasons of security, as well as to avoid unwanted publicity, all U.S. officers have been kept on a very tight leash, strictly confined to the bases where they teach or work except in special cases where limited R and R has been allowed on well-protected Pacific beaches or at one of the capital's most secure luxury hotels.

That has meant that the 14 officers teaching Salvadoran pilots and mechanics the ways of the Hueys are pretty much grounded at the military airfield at Ilopango. The five-man naval group trying to make shipshape the nation's fleet of four leaky patrol boats is based at naval headquarters at La Union on the Gulf of Fonseca. And the 15-man Special Forces contingent training a new 1,000-man heliborne immediate-reaction force are hunkered down at a new base being built around a former teachers' college about 29 miles west of the capital.

The 20 officers assigned to San Salvador -- 10 in the U.S. Embassy's military group handling liaison and logistics, the others at the general staff headquarters -- live in well-guarded villas and a luxury hotel annex sheltered behind high walls from which they only emerge in armored bullet-proof Cherokee vans.

Their military group's commander, Col. Moody Hayes, refuses any contact with journalists, even on "deep background," unless, he says, he is ordered to talk to them by the Pentagon.

But at one specially arranged meeting in a quiet villa in the capital, two of the Green Beret officers did discuss their mission guardedly the other day. And not unlike their lineal predecessors' praise for the South Vietnamese tigers under their tutelage, today's trainers had nothing but praise for the Salvadoran soldiers whose skills they are seeking to improve.

"I'm really super impressed with the moral of the troops," said a fit young officer talking of the trainees for the new heliborne brigade that will be formed when the training is completed 10 weeks from now. "They are well disciplined, they react to orders with no hesitation and do what they are told. They are very motivated."

An older, more senior, officer noted, however, that one problem they had encountered was a shortage of trained Salvadoran officers at a time when El Salvador is trying to expand its Army. "Too many of them have been cut out of the system by purges."

All Army generals and a number of colonels were retired when a group of reformist young officers and like-minded civilians overthrew a military dictatorship in October 1979. Additionally, the government maintains that hundreds of officers and soldiers have been dismissed for abuses, although they have declined to provide lists of those disciplined.

The U.S. Special Fores contingent is divided into three five-man teams, each of which takes on a Salvadoran infrantry company, normally about 150 men, for a two-week basic course in small-unit infrantry tactics.

Aware of the criticism leveled at the armed forces here because of a reported propensity to take no prisoners and to abuse the civilians they are supposed to defend, the younger officer said that part of the two-week course included a discussion on "treatment of prisoners and suspects."

"It emphasizes the necessity of humane treatment of both suspects and prisoners," the young officer said. "We insist that they understand the difference between the two: the suspect may not be an enemy and nothing should be done to make him one and even the prisoner should be humanely treated."

Can the Salvadorans accomplish their mission of beating the leftist guerrillas in the wild volcanic countryside with the U.S. training received and the military aid provided?

"That depends on many variables of which we do not have the answers," the senior officer admitted. "It will depend on how much stuff keeps coming over the borders from the Communists to the guerrillas. It will depend on how much the El Salvador armed forces keep getting from us. It will also in the end depend on the spirit and determination of each side."

Being military men, the two officers naturally enough said that El Salvador's forces could use more training than has now been arranged and, more military aid.

The Hueys, some of which Salvadoran pilots trained at Ilopango are already flying in medical evacuation and supply missions in the field, will improve the Salvadoran forces' mobility -- something that leaders of the guerrilla movement abroad admit they worry about. But the improvement will be marginal at best.

As one senior U.S. diplomat here has pointed out, the military aid provided first by President Carter in January and increased by President Reagan in March was never considered to be sufficient to allow the Salvadoran government to win the war.

"It was only enough to give the government a breathing space. The war cannot be won militarily, it must be won politically."

The two U.S. Special Forces officers, like their fellow officers here, are on strictly limited six-month tours of duty that will expire this summer. Will others come to replace them?

The administration and Congress will have to determine that. But whether or not more advisers come and more aid is granted, the growing "Made in USA" character of the Salvadoran armed forces seems bound to endure for a long time to come.