From the Topps Wikipedia page:
In 1951, Topps produced its first baseball cards in two different sets known today as Red Backs and Blue Backs. Each set contained 52 cards, like a deck of playing cards, and in fact the cards could be used to play a game that would simulate the events of a baseball game. Also like playing cards, the cards had rounded corners and were blank on one side, which was colored either red or blue (hence the names given to these sets). The other side featured the portrait of a player within a baseball diamond in the center, and in opposite corners a picture of a baseball together with the event for that card, such as "fly out" or "single".Instead of just choosing one team to finish my project, here is a different 1951 Topps Red Back for each of the six teams he played on during his career.