For me the 1950’s are notable for four developments:
The dominance of Simplex and its cheap and cheerful 1948 Simplex Tour de France pull-chain derailleur and its many variants. Simplex was a serious manufacturing operation, making more derailleurs than everyone else combined. For all that people now like to think of the Campagnolo Gran Sport as the definitive derailleur of the decade (see below) it should not be forgotten that even Simplex’s Italian factory in Milano produced more derailleurs than the whole of the rest of the Italian derailleur industry including Campagnolo.
The development of the 1951 Campagnolo Gran Sport, establishing the parallelogram derailleur as the high-end technology of the future. In the period 1951 to 1982 inclusive, Campagnolo produced variations of the basic Gran Sport design. In those 32 years Campagnolo was ridden by the winner in 23 of the Tours de France - an incredible record.
The appearance of the mass-market parallelogram derailleur in the form of the 1958 Huret Allvit. This opened the door to the concept of derailleur bikes as products suitable for everyman.
The awakening of the Japanese derailleur industry. Although Sanko had made a derailleur as early as 1947, it was during the 1950s that the industry built up a head of steam, and crucially SunTour (1955) and Shimano (1956) first entered the fray.