The original dies for the 1915 Panama-Pacific International Exposition Gold Dollar that were sent to the San Francisco Branch Mint by the Philadelphia Mint were missing the "S" mint mark to indicate their being coined at San Francisco.
As the San Francisco Branch Mint did not have the facilities to produce/update the dies at the time, the Gold Dollar dies were sent back to Philadelphia to be updated before they were used in San Francisco for production.
Unlike with the 1925 Fort Vancouver Centennial Half Dollar that was struck at the San Francisco Branch Mint without the "S" mint mark, the Pan-Pac gold dollars were not struck until their dies correctly reflected their origin facility.
You can find the "S" on the Gold Dollar's reverse under the denomination.
1915 Panama-Pacific International Exposition Gold Dollar
