Finally tracked down the correct model of this derailleur.  Based on the body design and the white jockey wheels this appears to be the Euclid rear derailleur.

Double adjustment for safe, efficient changing.

The hardened steel adjusting screw presses on a mobile steel bracket which varies the preload of the tension spring. The less the spring is preloaded, the tighter the chain wraps the freewheel, thus promoting quicker more accurate shifts. Between the mobile steel bracket and the body of the rear derailleur is a special anti-friction bushing which significantly reduces pivot friction and ensures the precise, flowing movement of the rear derailleur.