the 1951 sets which came in Red and Blue backs were actually a game and not considered collectible trading cards.
Quote: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".
Topps changed its approach in 1952, this time creating a much larger (407 total) set of baseball cards and packaging them with its signature product, bubblegum. The company also decided that its playing card model was too small (2 inches by 2-5/8 inches) and changed the dimensions to 2-3/4 inches by 3-5/8 inches with square corners. The cards now had a color portrait on one side, with statistical and biographical information on the other. This set became a landmark in the baseball card industry, and today the company considers this its first true baseball card set. Many of the oil paintings for the sets were rendered by artist Gerry Dvorak, who also worked as an animator for Famous Studios. In 1957, Topps shrank the dimensions of its cards slightly, to 2-1/2 inches by 3-1/2 inches, setting a standard that remains the basic format for most sports cards produced in the United States.It was at this time Topps began to use color photographs in their set.