Posted By:
Al C.risafulliBrian:
I started - and abandoned - a project about two years ago where I was trying to catalog all the different backs. There are more back variations in this set than most people think.
The blank backs are the most common - about half all the W502s I found had blank backs. Initially, I was of the belief that the blank backs were not even W502s but part of some other set, but I've come around to think that, in the game you're supposed to play with these cards, the blank backs are outs.
I'm going on memory here, but if I recall, of the printed backs, there are one baggers (most common), one bagger "hold what you got", two baggers, two baggers with prize redemptions (I think), three baggers, three baggers with prize redemptions, home runs, and home runs with prize redemptions.
My gut tells me that the way I listed them above is probably the order of scarcity.
THEN, the backs can all be found with the printing running from the bottom of the card to the top, and also from the top to the bottom.
That's seventeen back variations, for anyone counting.
On top of that, I believe that there may be different prize redemptions.
You could spend a lifetime trying to collect this set. As much as I love solving the puzzles, these are scarce enough that I felt there was no way I was ever going to get answers to all the questions I had. So I sold most of mine and started collecting Henry Johnson Confectioners instead. 
-Al