This patent describes the Selectic derailleur. This was a three speed device that was manufactured in volume.
The Selectic was notable because it had no lever or control cable. Instead it included a cunning and sophisticated mechanism that was operated by back pedalling. This patent is largely concerned with describing this mechanism. I am still trying to work out exactly how, when you are in the middle gear, the Selectic decides whether to shift up or down - the answer is in this patent but I have not quite understood it yet.