The San Francisco Mint was opened in 1854 to help process precious metals during the California Gold Rush. WIth huge demand increasing the SF Mint had to find a bigger building and in 1874 construction was officially finished of the 2nd location for the US Department of Treasury. Operations at this 2nd location now known as the "Old San Francisco Mint" were so large that at one point in 1906 the building held $300 Million in bullion (value at the time), fully one third of the entire United States gold reserves.
In 1962 the Mint of The United States At San Francisco was officially changed to "Assay Office" to accommodate growing demand from the public to exchange their silver certificates to the actual physical metal. The exchange program ran from March 1964 to June 24, 1968. This was one of those examples.
Rarely do you come across a "grease bar" with the original "United States Assay Office At San Francisco" hallmark because only a very limited amount of the first batches of bars had them. Quickly the demand picked up and grease bars were left blank as they rushed to satisfy the obligation to redeem silver certificates. Not only is it rare to find with a hallmark like this but also unique to find such a well struck example. Preserved for well over 50 years this piece even still has the original "grease bar ink" weight written on the bar from the time of issuance.
Truly a piece of American history from a time prior to dropping the gold standard when the US Dollar was sound.