US Assay Office San Francisco
In 1962, the status of the Mint Of The United States At San Francisco was officially changed from mint to assay office. The United States Assay Office At San Francisco operated until March 31, 1988 when mint status was restored. Silver ingot production at the United States Assay Office At San Francisco was concentrated on 100 oz class bars produced for exchange of silver certificates; the exchange program ran from March 1964 to June 24, 1968.
Ultimately these bars would come to be know as Grease Bars. As the demand for silver bullion overwhelmed production capability and standard United States Government precious metal ingot markings fell by the wayside, grease pencil markings of the weight were the only identifying mark.
Originally produced with United States Assay Office At San Francisco oval hallmarks, the first two pictures represent some of the finest early bars I have seen, the first with USAO hallmark , fineness and very low serial number 35; the second with USAO hallmark, fineness and weight. The No. 35 bar still shows the original grease pencil weight marking.
The next picture is an excellent representation of a true Grease Bar; no hallmark, no fineness, no weight and no serial number, only grease pencil marking of the weight.
On October 12, 1967, the Treasury Department issued the below notice which explains the appearance of 996 to 998 fineness rather than 999.
And finally, there are quite a few Grease Bars in circulation with other markings like the one below. United States Government markings are standardized and easily identified, most of these secondary markings are privately stamped.