Not All Silver Dated 1964 was struck in 1964! : eBay Guides

While most collectors believe that 1964 was the last year that the United States Mints struck 90% silver coins for circulation -- 1966 was actually the last year of the 90% silver coinage output for circulation!   Specifically, 1964 dated 90% silver coins were produced from January 1964 through April of 1966!

Information obtained from ConecaOnline.org website