U.S. National Archives (Record Group 104, Entry 235, Letters Sent by Director's Office) Pubblico Deposited
Record group 104, entry 235 covers letters sent from the Mint Director's office, covering the period 1873-1938. The recipient might be a specific mint, the Secretary of the Treasury, a foreign government, U.S. Department or Agency, or individuals and companies. This material currently (6/2016) resides at the National Archives facility in College Park, MD.
Roger W. Burdette notes on entry 235:
Letters sent by the Director’s office, 1873-1938. Press copy books. No reliable index.
Vols. 1-85 are large size fair copies 1884-1901; archives locator is RG104, 450/66/28/04.
Normal volumes are identified by calendar year and offices or subject covered, such as: San Francisco Mint 1890.
National Archives catalog record is https://catalog.archives.gov/id/7064779. See also the National Archives & Records Administration numismatic finding aids at https://nnp.wustl.edu/library/archivedetail/538870.