In a sense, the first Topps Traded set also happened to be its first baseball card set. In 1951, Topps released its inaugural 104-card baseball set in two different series consisting of 52 cards each--Series "A" and "B". Series "A" has come to be known as the Red Backs, while Series "B" has come to be known as the Blue Backs. It was in Series "A" that Topps introduced two Traded cards, signifying an update to the team status of the represented player. In the mail today, I received one of those Traded cards from the 1951 Topps Set.
Tommy Holmes' regular-issue '51 Topps card features an Outfielder for one of the final Boston Braves teams that would eventually relocate to Milwaukee after the 1952 season--long before moving to Atlanta in 1966. Apparently, sometime during the production of "A" series cards in the 1951 Topps set, an updated version of his card was released to reflect Holmes' changed status as the Manager of the Braves' Minor League affiliate in Hartford.
The other Traded card in the 1951 Topps set belongs to Gus Zernial. To my knowledge, the only other set that had Traded cards of this sort prior to 1972 is the 1969 Topps set, which also had two Traded-type cards. Of course, it is well recognized that the 1972 Topps set contained the actual first Traded cards, but those cards were numbered as part of the base set. The 1974 and 1976 sets were the first Topps Traded Sets to be numbered separately from the base set. Topps didn't begin manufacturing Traded sets regularly until 1981.