This is the first year in which the Mint struck 90% Silver Proof Kennedy Half's with an S mint mark. The last time the Mint had struck 90% Silver Proof Kennedy Half's was in 1964. However, the 1964 Proofs did not contain the S mint mark.
Additionally, the new Proof Silver Kennedy Half's struck from 1992 and after had slight differences in the details of the design. The 1992-S Silver Proofs had the lettering thicker on both the obverse and reverse than that of the 1964 Proofs. The Proof Silver Kennedy Half's have been struck by the U.S. Mint and sold in Silver Proof Sets every year since 1992.