Here's one from 1971 that I discovered a number of years ago. It's the Coins Checklist, #161.
Although you rarely see it listed as such anywhere, many people knew this card came in two versions because the card number on back was either positioned 'high' or 'low' (two noticeably different placements) and the fronts were slightly dissimilar (explained below). But when I studied these cards, it soon became very apparent that there were, in fact, three total variations:
1. Beneath the coins graphic, a solid black line is present. It 'completes' the white box.
2. In that same area, the black line is not present and the blue from the coins forms a straight edge across the gap.
3. The black line is not present and the blue from the coins is cropped differently, forming three distinct curved edges.
For those of you keeping score, the straight blue edge version has the card number in the 'high' position. The other two both have it 'low.' None of these cards seem to be any rarer than their counterparts.
And to
Mark, Brooks Robinson appears listed on the back of this card, so it may fit into your ultimate Brooksie checklist.