Like many other traces of World War II, one wouldn't know it was there today except for a few telltale signs.

From the outside, it seems nothing more than a large manor home, standing at the end of wooded Wasserkaefersteig Way, unassuming save for the barbed wire fencing and the armed sentry at the driveway gate. A posted sign also suggests more inside -- or as things turn out, underground -- than meets the eye.

"U.S. Mission -- Berlin Document Center," the sign says.

This is where the original Nazi files rest, all the membership cards and personnel dossiers and many of the administrative orders that belonged to Hitler's finest. The collection, 25 to 30 million bits in total, constitutes the last major German war relic in Berlin still in American hands.

Now West Germany, arguing it is time to demystify the records, wants them back. But the United States isn't taking any chances that some page may have been overlooked.

The Reagan administration intends to have the whole collection microfilmed -- the 10.7 million Nazi party membership cards, the 660,000 Schutzstaffel (SS), or special police, files, the 400,000 Sturmabteilung (SA), or storm troops, records, the 1.5 million pieces of party correspondence, the 2.5 million German citizenship claim reports, and more -- to give the U.S. National Archives a duplicate before ceding the originals to the Bonn government.

"This is the most interesting file we have," said Dan Simon, the center's director, showing a visitor into a downstairs room crowded with closely stacked notebooks that appear to stretch on for a city block or two. "What you're looking at now was the beginning of the master race."

Inside the notebooks are photographs, character reference sheets and meticulously detailed family trees for about 300,000 SS officers and their wives.

"If an SS officer wanted to be married," Simon explained, "he had to submit these applications tracing his and his wife's ancestries back 200 years. This was part of Hitler's effort to come up with pure Germans."

The bulk of these and other records are stored in thick-walled concrete chambers located two stories underground in a bunker that the Nazis used as a wiretapping center. The files themselves were brought here only at the war's end for sorting and cataloging, after being uncovered by Allied troops in various hideaways in Germany.

A sense of the old Nazi order lingers in the austere, solid shelter. "It's eerie as hell down here sometimes, especially at night," observed Simon, stepping out into a hallway. "You can almost hear the jackboots echo."

An agreement to transfer control of the center to the West German government was negotiated in 1980, but the United States, though sympathetic to Germany's wishes, has been seeking more time.

The technical and financial problems of arranging for completion of the microfilming are generally cited as cause for the delay. But one knowledgeable American source reported that political considerations, in connection with the Justice Department's current special investigation of suspected Nazi war criminals, made the Reagan administration hesitant to carry out the sensitive turnover of the center.

The Justice Department search, by the Office of Special Investigations (OSI) created three years ago to prosecute criminal fascist refugees from World War II, is expected to run for several more years. So far, by relying heavily on documents at the Berlin center that are cross-checked against U.S. immigration records to track down suspects, the agency has bought 25 cases to court seeking revocation of citizenship or deportation. Another 210 cases are considered active.

"We use the Berlin center almost every day," said Allan A. Ryan Jr., director of justice's OSI, in a telephone interview. "It's of great concern to us that we continue to have complete access to those records." At the same time, Ryan stressed he was not opposed to the center's transfer under the agreed terms.

West Germany had expected to take control of the facility this year or next. "We don't understand why it is taking so long to start the microfilming," said a Bonn government official familiar with the negotiations. "Perhaps the cost of the project has been a hurdle."

Simon, the center director, said sealed bids from four commercial copying companies have been taken and the microfilming could go ahead early next year. But, he added, the job could take three years to complete.

Some administration officials are under the impression that the West Germans, in fact, are in no hurry to receive what could be viewed as a political "hot potato." But the Bonn official, who asked not to be identified, sounded eager for the transfer to take place and said his government's patience would give way if the United States was found to be delaying intentionally.

It is to dispel the mystique of the place and, hopefully, put an end to speculation of dark secrets within the files that the West German government is interested in taking title to the records.

"We don't expect any sensations would come out," the West German official said. "Everything in the files is more or less known already. But we would hope that, by taking control, all things thought secret in there that have given rise to rumors would be stopped."

Even if all its many, neatly stacked and yellowing papers hold little additional mystery, the Berlin center remains an important and frequently used resource for many clients. Each month, about 3,000 to 4,000 requests for information arrive. This figure has been constant, Simon said, for the eight years he has run the facility.

About 65 percent of the requests come from West German agencies, checking on someone's citizenship, insurance or pension claim. All governments considered "friendly" to the United States are granted access to the files, Simon explained. So are scholars with institutional accreditation.

Requests from Communist governments aren't likely to be answered. The Soviets have never tried, Simon recalled, but the East Germans did several times and were refused.

As a result, however, of the cooperation given by Soviet and other East European authorities to the Justice Department investigations, Simon said he recently recommended to the State Department that some Communist governments be allowed to draw on the center.

Off in a corner of the facility, in an upstairs room that may, because of its above-ground location, seem a bit more generous than those cells down below, are the records of the hundreds who were tried for opposing the Nazi party. Leafing through a few of these fraying, increasingly fragile portfolios, Simon observed: "This is the only place where you could say it is an honor to have a file here."