After 1964, no Proof Sets or Mint Sets were issued until 1968.  In the interim, the U.S. Mint issued what was called a Special Mint Set.  These sets were similar to a Proof Set in that the 5 coins tended to have mirror surfaces, but not to the brilliance normally found for a true Proof Set.  The preparation of the planchets and dies used for these coins did not meet the strict definition of that required for a PROOF designation. Nevertheless, these coins are quite beautiful and distinctly different from that of regular issues for these years.