It also was the last vestige of postwar four-power rule in Germany. Guard details of 100 soldiers each from Britain, France, the Soviet Union and the United States continued in monthly rotation even after the sickly, aged Rudolf Hess become the only occupant. Guards manned the towers and cellblock monitoring stations even during Hess's periodic hospital stays.

To make room for the original seven prisoners, some 600 common criminals were removed from the fortress-like structure in the British sector of Berlin. The only improvements since 1871 were then made to tighten security with an outside 15-foot, barbed-wire fence, guard towers and search-lights. The prison also received a fresh coat of paint.

Inside the fence was a 28-yard ''dead space'' in which even guards were forbidden to walk at will. Access to the prison was through a guardhouse, which itself formed part of a 15-foot inner brick wall, and the Spartan cells for the seven inmates were on a single floor, in the building behind the wall. A Severe Regimen

There also were two small yards that served as the prisoners' access to the light of day.

Glaring lights and an eerie silence, broken occasionally by birdsong or a plane's drone overhead, prevailed. The rules, at first Draconian, were relaxed slightly over Soviet protests in the 1950's to maintain the prisoners' health and sanity.

Even so, the regimen remained severe, with letters or visits few and far between, sparse and strictly limited reading matter, and no talk permitted among prisoners, or between them and the guards, except under special circumstances.