On both 1992 Topps and Topps Gold football cards, there are several variations on the back of the card, where the copyright line appears.
Here is the front and back of Herschel Walker's Gold card:
If you look on the bottom left of the card back, you'll notice a capital letter, C, right under the Topps logo.
Here it is blown up:
However, there is another variation in which there is an asterisk to the right of the C. See below:
Apparently, the cards in these sets can either be A, B, C, D, E or A*, B*, C* D* or E*. Herschel's cards only have the C and C* varieties.
I've tried to find out why they were produced this way, to no avail. Probably something to do with production runs.
As any single player collector knows, you'll never have every card produced for a particular athlete, the One of one cards alone, dictate that. However, these are inexpensive cards and once I became aware of the variation (thanks, as always, TCDB). I quickly picked up the version I didn't already have in the Hoard. (I already had the asterisk version).