[Arran]

Peter;
  Shadow meters, or shadowgraphs, pre dated magic eye tubes, I believe that Philco introduced them in 1932-33, though they were not the only manufacturer that used them, though they may have been the last. Philco never used Magic Eyes on any of their domestic U.S, or Canadian models. Magic Eye tubes debut in the 1936 RCA models and were a bit of a craze for a time, and it was an easy way to add an extra tube to a set without really changing the circuitry. Manufacturers would add a magic eye to a five tube set, and market it as a six tube, or add it to a six tube set and advertise it as a seven tube model, all the way up the line. Another scheme was to use a separate 1st audio tube (triode) and second detector tube (double diode) which really add nothing to the performance of the radio, especially if only one diode is used, a waste of electricity.
Regards
Arran