Campagnolo Corsa rear derailleur was made during 1946 - 48. The first postwar version had two shift levers, and the shifting fork was on the top run of chain.It required back-pedaling to shift. To shift gears, the rider reached back and turned the upper lever to unlock the rear wheel quick release. Then he pedaled backwards and turned the lower lever to move the lifting fork against the chain, until the chain jumped to the next sprocket. The wheel moved backwards by its own weigh. On upshifts, the wheel was pulled forwards by the chain. Then he pedaled forward lightly so that the chain was not stretched too tight. Finally, he turned the upper lever back to lock the quit relase.