100 oz class silver ingots produced by the United States Assay Office San Francisco are fairly common; most have no markings and are the true "grease bars" with only the weight marked in grease pencil. Early productions were hallmarked, serial numbered and stamped with 999 fineness. As time passed and demand increased, production ramped up and markings fell by the wayside. The U.S. Treasury Department letter pictured, announces the reduction of fineness to .996 and .998 as of November 1, 1967. The ingot pictured is a very rare .998 example with hallmark.  For more information on grease bars, check out SILVERINGOTS.COM