1991 United States Proof Set, 5 Coins Inside!. Original packaging: The coins of the 1991-S Proof Set were placed in individual holes in a purple-colored, textured paper and cardboard insert, then sealed in a two-piece, clear plastic case. The case was then inserted into a protective outer cover, consisting of purple cardboard, imprinted with the somewhat confusing silver words: "United States Mint Proof Set • 1991." The new collector might ask, "Is this a Mint Set or a Proof Set?"