1953- Readex converted into microprint "many, but not all, of those United States Government publications not designated to be sent to depository libraries, consisting "of approximately 12,000 items annually, which, for the most part, are not printed by the Government Printing Office, but are produced independently by the issuing agency ..."--Readex Microprint Corp. catalog, 1979-1980."
2-3 year time lag between listing in Monthly Catalog and receipt of microprint.
1946-September 30, 2004 Library of Congress assisted libraries in "obtaining documents that were not distributed through the Federal Depository Library Program, not for sale by the Government Printing Office, and not available directly from the publishing agency." Wikipedia Initially started after WWII to distribute formally classified documents.
Regional libraries paid annual subscription to get non-GPO documents. "Suggestions to GPO" from Federal Documents Task Force, ALA GODORT, 2/5/1973 Documents to the People, vol. 1, no. 3, May 1973, p. 25
Proquest Statistical Insight
"ASI indexes and abstracts statistical data published by all branches and agencies of the federal government, including both depository and non-depository publications." Lexis Nexis Statistical Insight Guidebook, p. 6
Example: January, 1976, no. 972, How to Order Publications, p. 6 and Current Population Report Series P-20 entry 00173, p. 28. "Marital Status and Living Arrangements" 1974 (Current Population Reports: P-20 Population characteristics. no. 27)
In Basic Search, type "current population reports p-20." Sort by date to get to 1974. Record Number: 1974 ASI 2546-1