Last month the Library of Congress put out a newly revised finding aid for its German captured documents collection. The finding aid includes a brief history of the collection including the process of returning originals to Germany and the retention of microfilm copies.
The new finding aid is a revision of an earlier one from 1989. It notes that for many years before that the "sole access to the collection was Gerhard L. Weinberg's Guide to Captured German Documents (Columbia University, Bureau of Applied Social Research, 1952)." Weinberg's guide is available in full-text online courtesy of the HathiTrust Digital Library here - it was produced following WWII as part of the War Documentation Project (WDP), which was "conceived to answer a long-standing requirement for the systematic research exploitation of the vast masses of captured documents which came into the hands of the United States Government." The WDP was part of the Bureau of Applied Social Research at Columbia University, which continues to hold the archives of the WDP.
Weinberg's guide was also not confined to the Library of Congress collection - it includes information on German collections in, among other places, the "Hoover Institute and Library" - and was part of the WDP's efforts "to secure as accurate information as possible on the fate and location of freely accessible German documentary sources scattered by the fate of war."