Official United States Mint Sets sold by the Treasury from 1947 to 1958 contained two specimens of each regular issue denomination and mint mark, called “double-mint” sets. Since 1959 official U.S. mint sets have been sealed in a protective plastic envelope. In 1965, 1966 and 1967 higher quality mint sets were made as a substitute for proof sets (proof sets were not produced during these years). The 1966 and 1967 sets were packaged in hard plastic holders. These mint sets were called SMS, or, Special Mint Sets.