I found three of these in the theatre I work in. The theatre itself was built 1926-1928 (it took ~2 years), and the receptacles were in the projection booth... still wired with the original wiring. I would say the 1928 guess is the best bet.
As for 1920's wiring, anyone have any online resources they really like? I'm finding a bunch of oddities, and would like to know their history.
