Gino Bartali, a conservative man who saw himself as a skilled craftsman, loved the intricasies and complexities of Tullio Campagnolo's Heath Robinson rod-operated gear changers like the infamous Campagnolo Corsa. He used one to win the 1948 Tour de France.
Fausto Coppi, however, revelled in modernity, progress and technology - and despised Tullio's ornate contraptions. In 1949 Fausto Coppi defected from Campagnolo to Simplex, and horror of horrors, won the Tour de France. An Italian demi-god had triumphed using French equipment. Tullio Campagnolo was humiliated and mortified.
But the shock was exactly what was needed to get the, ever-cautious, Tullio off his backside and spur him to produce, in 1951, his signature Campagnolo Gran Sport derailleur. It, famously, adopted a parallelogram mechanism, and its strength and accuracy defined the way that derailleurs operate up to this very day.
For the 1952 season, Fausto Coppi returned to the Campagnolo fold and won the Tour de France by a trifling 28 minutes. For the next 50 years Campagnolo was the unchallengeable king of the peleton, although perhaps not always the unchallengeable king of consumer markets.This particular example of a Campagnolo Gran Sport has:
I think this indicates that it dates from around 1961.