On the 1964 Special Mint Set
All known 1964 SMS coins have a satiny surface, strong details, and a sharp rim. A lack of contact marks and sometimes die polishing marksidentify these coins as being specially struck. Furthermore, they do not display the mirror-like reflective surfaces of Proofs, yet the strike isclearly stronger than either the 1964 Proof coinage or the SMS coinage of 1965, '66, and '67.

The coins were unknown to the numismatic community at large until the 1990s. For many years, the first appearance at auction of pieces from a 1964Special Mint Set was thought to be 1993, but researchers later discovered an unusual five-piece set of coins dated 1964 and struck at thePhiladelphia Mint was offered at auction in 1991. The five Special Strikes in the '91 sale were originally contained in the same plastic coinholder.

Numismatists believe that the 1964 Special Mint Sets may have belonged to former U.S. Mint Director Eva Adams, who was instrumental in bringingthe Kennedy half dollar from concept to circulation in just a couple of months after the assassination of President John F. Kennedy on November 22,1963.

The former Mint director died on August 23, 1991, and her estate was sold soon after. Coin dealer Lester Merkin is known to have bought some ifnot all of the coins from that sale.

For more information on the 1964 "Special Mint Set" Coins, see this PCGS article from 2008:
1964 "Special Mint Set" Coins(https://www.pcgs.com/news/1964-special-mint-set-coins)