U.S. Mint Sets were produced from 1947 to date.  From 1947 through 1958, each mint set contained 2 of each denomination from each mint, and the coins were housed in cardboard sleeves, and delivered in a brown envelope.  These sets are quite scarce, and the coins tend to be attractively toned.  If you love toned coins, these very scarce and desirable sets are for you!