Below is a pictire of a Vittoria on a bike. You back-pedal and twist the triangle on top to move the butterflies attached to the chain stay to change gears. The lever is then used to adjust chain tension. There are a couple of versions starting in 1933 and loosing favor by the war.

The jockey wheel on the OMAC is attached to a rod that is in a sleeve with a spring. That goes up and down to keep chain tension. You twist the lever of the seat stay to move that jockey wheel from side to side and change gears. Nice in theory and I haven't really tried too hard, but I have yet to shift it while in motion. It is from the late 40s/early 50s and is a knock-off of a Campagnolo cambio corsa or Paris-Roubaix.