Superior Electric is located in Bristol, Ct. The company has been producing variable transformers (Powerstat) since 1938. Later they started with the production of synchronous and stepper motors, followed by a full range of drives and controls.
The brand name of these motors and drives is Slo-Syn
This page contains a part of my collection of historical Superior Electric documentation, converted to PDF. These catalogs are from 1968 to 1975. You can find a lot of very famous products, like STM1800, NCI, MNC, LNC, PNC, TRP125 and even a financial report from 1967.