U.S. National Archives (Record Group 104, Entry 235, Letters Sent by Director's Office)
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.