Originally Posted by luker

Helps a lot. Older simplex derailleurs and a wealth of others used a double rear lever, like the one in your picture. The idea was that the main lever positioned the derailleur, and the secondary lever adjusted the chain tension to a minimum. It was thought that excessive chain tension was terrible. The idea faded away in the early 50's. I have a couple of those derailleurs, but have never found a double lever, except in pictures.