Which specific Campagnolo model is cited as the first commercially successful derailleur in 1949?

Gran Sport.

While the concept of the derailleur existed since 1895 and the Tour de France mandated its use by 1937, the necessary engineering refinement for mass commercial viability took until after World War II. The company widely credited with perfecting this modern, practical system was Campagnolo. In 1949, they introduced the Gran Sport model. This component is frequently cited as the first commercially successful and truly practical parallelogram rear derailleur. Its success stemmed from combining effective chain retention with a relatively straightforward cable actuation system that could be operated using a lever positioned on the bicycle's downtube, making it reliable enough for the serious cyclist market.