THE ONE-MAN checkpoint on the southwestern fringe of West Berlin has no catchy name, and few camera toting sightseers manage to find their way there.

There isn't much to it, anyway. A lone cop, a roadblock that can be raised, a bicycle stand with a few two wheelers. And the inevitable four-language billboards of the divided city: 'You Are Leaving the American Sector."

Beyond that, a dirt road runs through swampy meadows and sparse pines and the "death strip" to another checkpoint, this one manned by the East Germans.

At the other end of the road — blocked in by coiled barbed wire, concrete obstacles and a wall — lies Steinstücken, a tiny chunk of West Berlin surrounded by East Germany.

It is 5:30 p.m., about the time when many residents of the 30-acre exclave return from their jobs in the city.

Many drive their own cars, but some use the city bus and then mount the bikes they have left in the trusted care of the police guard to pedal to their homes less than a mile across the border.

The German policemen posted here usually raise the bar well in advance of approaching cars and are thanked with a brief wave.

"We know almost everyone in there by name, and they know us," said the guard, who pointed the way up to the crest of a hill which offers a good view in the direction of the exclave.

THE POPULATION of the "island" of Steinstücken is about 180, a third of them pensioners and more than 40 children. That's an appreciable gain over the 1950s when only 120 people lived there, and it reflects the relative feeling of security among residents.

This security is guaranteed by a three-man U.S. Army military police detail stationed in the isolated village since 1961 when Gen. Lucius D. Clay (ret.), then President Kennedy's personal representative in Berlin, ordered that action.

Old Glory has a place of honor at the bar of Restaurant Steinstücken, which now serves as a sort of community hall. In happier days, Steinstücken was a popular weekend resort between Berlin and Potsdam, and the restaurant sometimes had 30 employees in season.

The former owner runs the hamlet's only business establishment, a small grocery store, and takes special pride in providing milk and fresh rolls by the 8 a.m. opening time. She has to get up early to bring in her supplies from the Zehlendorf borough on the other side.

THE GIs don't use the access road through East German territory. They and then supplies are flown in by helicopter, despite claims that this violates Communist air space.

It has become a tradition for the U.S. Berlin Brigade commander to personally inspect his Steinstücken "vanguard" occasionally. Clay used to do it, and when he left Berlin in 1962, he gave a television console to the residents. In an entry in the gasthaus guestbook, dated April 29, 1962, Clay wrote that Steinstücken was the only place in the world "where I enjoy drinking wine before lunch."

LIFE in the 36 houses of the exclave goes on under the constant surveillance of the East German guards. Visitors are extremely rare, and the only routine contacts with the outside world are provided through the mailman, the doctor, and, less frequently, the midwife and fire department.

Another connection to the West is the tracks of the Wannsee-Drewitz railroad that cut through the exclave, bringing in freight trains with goods for West Berlin. But there's no stop here; the crossing was unprotected until August 1965 when the Communists helped to install a footbridge in an unprecedented communal effort.

The Reds also provide the utilities — water, gas and electricity — but the Steinstücken telephones are linked up with West Berlin.

There are several other and smaller West Berlin exclaves — notably Eiskeller in the Spandau borough, British sector — but none of them has been in the news as often as Steinstücken.

In 1951, East German Vopos supported by Soviet troops attempted to annex the hamlet but were stopped by a sharp protest from the U.S. commandant.

The most serious confrontation came in 1963 when Red guards several times threw stones at the MP patrols, which retaliated by tossing tear-gas grenades.

SINCE then the presence of the MPs have become routine to residents and Communists alike, but the East Germans have put up a veritable fortress of watch towers, walls and barbed wire around the exclave.

At night, the area is lit up like a Christmas display and residents frequently complain of gunfire and flares.

But despite the isolation and other restrictions, Steinstücken's citizens are stoically holding out. Last year, German newspapers reported, there were even buyers from West Berlin and West Germany for houses in the exclave.