"S" Mint Marking. the United States Mint certifies that a Silver Proof Set of current United States circulating coinage was produced and packaged at the United States Mint, San Francisco, California. Each coin bears the mintmark "S" designation of the facility. United States proof coins are produced from carefully selected planchets, or coin blanks, that have been burnished to a high luster. The finished coin with its frosted cameo image on a mirror-like field is carefully inspected-with gloved hands to protect its surface-before being assembled into sets. The proof coins are then placed into transparent presention cases.