Claude,
This certainly is a complicated set. Here is how I think it breaks down.
There are only 2 issues. The first printing set of 55 and the second printing set of 99.
The first print cards are numbered 1-55 and have either an "A" or "B" suffix which relates to which puzzle the back belongs to. So cards 1-35 are all "A" and make one puzzle while cards 36-55 are all "B" and make the "B" puzzle.
Here is a back:
The 2nd printing cards are all numbered "X of 99 cards Puzzle Y". (Where Y is either an "A" for cards 1-35, "B" for cards 36-55, and "C" for cards 56-99).
Here is a back:
Doesn't sound too bad right? Well, since all the cards are numbered on the backs it is possible to find the same image on the front with a different number on the back. This is true for both the original 55 and the set of 99. I have no idea how many of the front/back variations there are.
Then of course you have all the different flavors from non-US companies.