SMS(Special Mint Set) half dollars were minted from 1965-67. You have a 1964-D Kennedy half that is a business/circulating strike coin.
In 1964 of special mint set was also applied Including a dime nickel quarter painting dollar half-and-half dollar
True, BUT, they did not have the "D" Mint mark. Your coin is NOT one of the 1964 SMS Kennedy's but simply a nice looking, to you, 1964-D Kennedy Half Dollar. Why do you ask?