FWIW, they're not 'plate position marks', but rather sheet codes. They are valid variations that can be documented here, much to the chagrin of some members. For typical Topps sets of the era (792 cards), they were printed on sheets of 132 cards (11x12), lettered A-F. There are many instances, especially in non-sports sets, of 66 card sets being available with either * or ** possible on each card as the sheet code. I assume those were for top/bottom half of the sheet. 1991 Topps had several cards with either E* or F* that had variations of E*F* (or A*/B*/A*B*, similar for CD), but it's rare to have more than two variations of the sheet codes possible for a single card. 1991 Bowman cards on the E sheet are the only ones I know of with more than two variation of the codes (E*-no space, E *-with space, and E**).
The tricky part is getting a complete list of all possible code variations so that the checklist only has to be updated once.