In 1965, 66, and 67 the Mint did not strike true proof coins. What they did was overpolish the dies to strike the planchets one time and the result was a coin which had the characteristics of a proof but without all of the extra preparations involved in making true proof coins. These coins were not handled the same as real proofs since they did come in contact with each other and you will see marks on some of them.
A proof coin is a highly polished planchet struck twice by highly polished dies, at least that's the way they used to make them it could be different now.
Ed
ANA LM-3175