Made by The Simpson Electric Company, Chicago Illinois. Model 59 AC Volt meter. Mount diameter is 2-3/4 (housing OD) and face is 4-5/8 x 4-1/8 inches. Untested. Plastic-type housing.
Ray Simpson was the president of Jewell Instrument Company, then went on his own around 1927, when he invented the indicating mechanism for the earth inductor compass used in Charles Lindberg's historic Spirit of St. Louis flight. Simpson gauges were later used in NASA Apollo mission spacecraft.
If not used for testing, think about your next industrial project, rat rod car project, or office and bar decor! I have other vintage meters listed, so take a look! I will combine shipping.