Special Mint Set?
I recall the Special Mint Sets made from 1965-67. Although they were not proofs, they were nice in their own way and provided a set of coinage that was the best the Mint offered in the first years of non-silver coinage. The sets were not that popular with collectors. The half dollar was 40% silver, no other silver, and the issue price was $4. Compare that to proof sets of the previous years, with three 90% silver coins, selling for $2.10.