For W. Berlin mayor, nipping across the wall is a big decision. Invitation to E. Berlin for 750th celebration fraught with symbolism

When West Berlin Mayor Eberhard Diepgen nipped through the Berlin Wall to the East recently to help celebrate 750 years of the Christian church in Berlin he may have been setting a trend. The French hope not; the British don't mind; the United States has misgivings but will let Mr. Diepgen make up his own mind. The West German government and West Berliners themselves have mixed feelings.

Diepgen has only another two weeks or so in which to decide whether or not he will attend the meeting of 120 mayors in East Berlin in June and the official East German celebration of Berlin's 750th anniversary in October. His office denies press reports that he has already decided to attend the East Berlin functions.

The current dither is the result of what is otherwise the festive occasion of Berlin's 750th birthday party, an ambitious celebration that will last all year long in both halves of this divided city. As part of it the East Germans are gathering mayors from around the world to discuss common municipal problems - and, some Western officials fear, to give subtle recognition to East German claims that East Berlin's status is different from West Berlin's.

Berlin was occupied by the four World War II Allies - the US, Britain, France, and the Soviet Union - in 1945. Since a German-Soviet peace treaty has never been signed - and since the three Western allies are responsible for the security of the enclave of West Berlin 60 miles inside East Germany - the four-power status of Berlin has been officially maintained for the past 42 years. It was formally confirmed in the ``Quadripartite Agreement'' of 1971.

The West interprets this to mean that all of Berlin, East as well as West, legally remains under four-power occupation. Even when it is East Germany that initiates challenges to the status of Berlin - as happened last year, when East German border guards began demanding that Western diplomats show their passports at intra-city crosspoints - the Western powers conspicuously ignore East Germany and appeal for redress directly to the Soviet Union.

The Soviet Union - while exercising its own rights in West Berlin in running military patrols and co-administering the Spandau Prison (with the elderly Rudolf Hess as its sole prisoner), and allowing the exercise of such Western rights as military patrols in East Berlin - says that East Berlin is in fact separate and is the capital of the (East) German Democratic Republic.

In practice, West Berliners administer West Berlin (under allied ordinances that are established with extensive consultation with the West Berlin government); East Germans administer East Berlin (which they call simply ``Berlin,'' with no qualifying adjective).

Nonetheless, the Western allies do not want to allow the legal status of West Berlin - and therefore the foundation of their ability to defend the city militarily - to be nibbled away. This is what the French in particular fear in any visit by the West Berlin mayor to an anniversary conference that treats East Berlin as the capital of the GDR, juridically divisible from West Berlin.

Many officials in the West German government in Bonn feel this same unease, but good East-West German relations are so important to Bonn that the government is already putting some pressure on the Western allies to approve Diepgen's visit as a goodwill gesture, according to Joachim Boelke, editor of the West Berlin daily Tagesspiegel.

After a decade and a half of no East-West crises over Berlin, many West Berliners find such legal niceties constraining. In particular, younger West Berliners like Mayor Diepgen find it irritating that the mayor of Stuttgart can take part in the East Berlin celebrations with no questions asked, while the West Berlin mayor has to walk on eggshells.

Older West Berliners, who remember the Soviet blockade of Berlin in 1948 and Soviet leader Nikita Khrushchev's threats during the Berlin crisis of the late 1950s and early 1960s, worry much more about what kind of legal precedent might be set if Diepgen does accept the East Berlin invitation.