There is no shortage of oddities in what your webmeister considers the golden (1949-57) or silver (1966-72) ages of Topps, especially the latter. One such item is this 2x2 proof panel I have of 1967 Topps baseball cards printed on thin and extremely high quality paper:
The "cards" are standard sized but the bottom has a little extra to it. The reverse is pristine white (that blotch is some dirt it must have picked up along the way). It really is there, so keep scrolling:
Looks like the proverbial polar bear eating mashed potatoes in a snowstorm, doesn't it? That panel was once Woody Gelman's by the way.
I would think these were part of the proofing process. In later years (mid 70's) I know there are proof cards printed on ultra white cardboard but the process may not have been 100% refined in 1967 so Topps used high quality paper. I have seen a couple other 4 card panels like this over the years, so they are not exactly common.