Originally Posted by
ThermionicScott
Another data point. The Retro-Velo timeline notes that Campagnolo revised their Grand Sport rear derailleur to handle 6-speed freewheels -- in 1961!
Velo-Retro: Campagnolo Timeline
The race managers probably said to Tullio, "Tullio, my old and dear friend! We must be able to test 6-wide rear sprocket sets. Will you provide something to support us?"