It really goes back further than that. The Mint only
acknowledged there was a difference (by calling them Brilliant Finish) after the satin finish was abandoned in 2011.
After the 1965-1967 SMS run, uncirculated mint set coins continued to be made differently. They were still minted on different presses (lower speed & higher pressure) with fresher dies and received better handling than typical business strike coins. This may have not been the norm, and some mixing may have occurred, but their distinction from regular business strikes coins has been certain since the the sets resumed in 1984 and the press lines were separated.
I am pulling this from memory. Cladking may come along and correct some things for me.
