A really short history lesson. Short because I am NOT an expert on this. I have only tried this once and that was only because Dan Ulwelling enjoyed watching my ineptitude. There have been rear derailleurs that we would recognize on touring bikes dating back to maybe the 1920s. Racing bikes were single speed and anyway these were not feasible for racing bikes. The early derailleurs were mounted forward of the freewheel and wrapped the chain around the freewheel cogs, making wheel removal difficult. With the rough roads and primitive tires, there were a lot of blown tires and speed was essential. Also, keep in mind the riders carried tires wrapped around their shoulders and were responsible for the changing the tire on the existing wheel, rather than have a a tire mechanic jump from a support car and slap in a new wheel.
As you read this, please keep in mind your finger-touch brifter indexed shifters. To shift the Cambio Corsa, the rider had to free the axle tension with the long lever so it could slide fore and aft, then move the chain to the proper freewheel cog with the shorter lever, backpedaling to shift to a larger cog - forward to shift to a smaller (I think), then retighten the axle in it's the new position. Having never mastered this, I can only compare it to double clutch downshifting and engine rev upshifting old straight-stick truck without synchromesh. Only ... I could actually do that.
This is Masini's newest find (more later). It is curious because Tullio Campagnolo invented this device in the mid 1940's, and later models, the early 50s Paris Roubaix, had just one lever. By 1952 Campagnolo came out with the Gran Sport derailleur, which most of us would recognize as "modern". This bicycle is apparently from 1955. Why was it made with, what was by then, an obsolete shifting system? Was it for an older racer that didn't trust the new fangled equipment? The 1955 date is based on the date on the bolts. Maybe this bike sat around, not built up, for three or four years? Whatever, it it very cool.
(Please correct as necessary.)