I was only kidding a little about the variation theme. Unlike the MBs, the 1982 Topps Blackless cards made their way into the hobby via packs.
But some check listed Topps issues did enter the market directly rather than packs. Most recently was the 2012 Topps Heritage Update, but there were numerous send away for a card or set of cards in the 80s. The only things I can think of pre 1980 were the 1973 Reprints of 1953 cards and possibly the 1971 Greatest Moments set. Some people say they came in packs but I am not aware of anyone coming up with a pack or wrapper so far ( there is one empty box I have seen at auction). Plus some variations may exist as a function of the appearance of cards only in factory sets.
As usual Carlton, a great tutorial on MBs specifically and the 68 set in general
I do agree on the 1967 Maris proof, but if you pursue "master" sets you have to have one
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