Another one from our vaults...
Formerly called the San Francisco Mint, it became strictly an Assay Office in 1962 until resuming coinage operations again in 1965. During this time precious metal bars would be submitted to the Assay Office to get the US government's stamp of approval on the purity of bars made by various manufacturers.
Unlike other countries, the stamp of a US Assay Office is not a "hallmark" but is simply an attestation of the weight and purity of the bar itself. The could have been made by anyone!