This explanation is from cardboardconnection.com:
"1969 Topps Man on the Moon was actually release in two separate issues. The first has 55 cards. The latter reissue, which was released in 1970, saw the checklist expand to 99 cards. This included the same 55 cards from the original release plus 44 new cards. However, the numbering is a little different. The initial issue and the reissue can be discerned on the card backs, even the first 55 cards. The first release had an 'A' or a 'B' after each number (1A, 2A, 54B, etc) to discern what puzzle it forms. For the reissue, all 99 cards are numbered 'of 99' (1 of 99, 2 of 99, 54 of 99, etc)."