That special mint set (SMS) was sold by the U.S. Mint in 1967 because they stopped making proof sets in 1965. They did resume proof sets in 1968. The holder is a Capitol Plastics holder. It is definitely not a slab. The materials used by the mint to house the SMS were inferior and the coins tarnish with age. The Capitol Plastics holder is inert and is a great way to store coins. It is made up of 3 pieces of plastic, two outer clear pieces and a middle piece that has holes cut into it for the coins. The 3 pieces are held together by plastic screws and can be easily unassembled if you want to touch the coins. These sets are common, not expensive, but they do look nice.