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@dicki
In the early to mid-1930s a rider probably had three sprockets in the back and one up front. Riding below the chainstay was a pulley wheel which acted as a chain tensioner. Early versions (as in the pictures below) required the rider to reduce the chain tension, pedal backwards and with his gloved hand move the chain to the desired sprocket. Later systems, called the Vittoria Margherita (1935 and on) had a rod-controlled pusher on the chainstay that would move the chain when the rider backpedaled.This is barely visible in the photo above. It was clumsy but it beat getting off the bike then removing, flipping and replacing the rear wheel. Cycle historian Frank Berto noted that the Vittoria derailleur systems were rugged, simple, reliable and the tension wheel rode high enough to give good ground clearance, an important consideration in an era of bad roads. Unlike the some of the more fragile gear changing system on the market at the time, the Vittoria still worked when fouled with mud.
http://www.bikeraceinfo.com/photo-galleries/derailleurs-1.html