Denver Mint records scanned at the National Archives (NARA) in Denver - covering the period 1909 - 1934. Record group 104.  Information resource name and date range: “Record of Deposit Received by Mail Express, Apr. 24, 1914 - March 30, 1920.”  Courtesy of John Graffeo.  

NARA description: This series consists of records of deposits received by express mail. The records include the dates the mail was received, the amount of the deposit, and a brief description of the transaction.

NARA catalog entry: https://catalog.archives.gov/id/2674936.