The 1st major baseball card set by Topps consisted of two different series of 52 cards each for a total of 104 cards in the complete set. Series "A" actually included two additional Traded/Update cards (Gus Zernial #A-36 and Tommy Holmes #A-52) bringing the total number in series "A" up to 54 cards--or 106 cards overall for '51T.
Packs of 1951 Topps sold for 1 cent for 2 cards in either series. The 2" x 2-5/8" card size was somewhat standard for its era, which was nearly identical to the size of the 1948, 1949, and 1950 Bowman cards. All four sets fit nicely into 12-pocket pages. Bowman actually increased its card size for the 1951 set.
I'd eventually like to see both series of the 1951 Topps set get combined on the database, as well as in price guides someday. We don't see series 1 and series 2 of 2016 Topps get separated into two different sets. I don't understand why series "A" and series "B" of the 1951 set gets separated? Well, actually I do know. Both series were numbered from #1-52, which probably made it easier for price guides to list them separately as two different set--but they are not. My suggestion would be to number the 1951 Topps set from #A-1 to #A-52, and then from #B-1 to #B-52. Take a look at the numbering for the 1936 Goudey Premium set, which looks very similar. The two variations can be numbered #A-36a and #A-36b, or #A-52a and #A-52b. There's your 104-106 card set.
Also, can this set get a little more respect on the Trading Card Database? At the time of this posting, the rating stands at 6.5 points. I guess that's just unbelievable to me.