U.S. National Archives (Record Group 104, Entry UD) Público Deposited
The central files of the U.S. Mint, 1898-1988. This material currently (6/16) resides at the National Archive in College Park, MD. The UD entry is not completely explored but contains information on many aspects of U.S. coinage development in the 20th century.
National Archives catalog record: https://catalog.archives.gov/id/5757372. See also the National Archives & Records Administration numismatic finding aids at https://nnp.wustl.edu/library/archivedetail/538870.