US Proof Sets
A proof set is a coin made for presentation or collecting purpose. Starting from second half of the 19th century, the Mint struck proofs of many of the coins. A proof set is made from an exclusively decidated die which is only used to mint proof sets. Such dies are cleaned more frequently than ordinary coins. Proof sets are inspected from uniformity and will be rejected if any defection is found. Proofs that pass inspection are put in holders, so nothing will interfere with the finish. So keep in mind a coin that has been circulated is not a proof no matter how perfect its condition is.
Proof sets have been made by the Mint since 1936. However no proof sets had been issued between 1943 and 1949. Beginning in 1968, they were issued in plastic holders rather than vinyl. Now proof sets are only struck in San Francisco Mint and they all have "S" mint mark.
US proof sets:
