[Penthode]

Actually the 925M scale is marked 21 to 99 but tunes 42.1MHz to 49.9MHz. The FCC assigned channels for the broadcasters for the old band and the new band. The old band FM channels were 21 to 99 and the new FM band channels are 200 to 299. The channel numbers fell into disuse in the late '40's.

As for a signal, I have made a hobby of converting old TV VHF tuners to FM old band to new band translators. A TV tuner IF output is in the 40MHz range, perfect for the prewar FM radios. Because the old band is 8MHz wide and the new band is 20MHz wide, the VHF tuner may be converted to provide three overlapping 8MHz bands eg 88 to 96MHz, 94 to 102MHz and 100 to 108MHz. I sweep align the 8MHz bands for each ensuring a good flat response with good gain. The good signal to noise afforded by the newer TV tuner works extremely well with the old prewar FM set: I find a properly restored prewar FM set generally with as much sensitivity as a new FM set and of course the sound is fantastic. It is a testimony to E H Armstrong how good FM must have sounded in 1941!

It is surprising how many prewar FM sets survive. (I have a Stromberg Carlson and two Capeharts and am looking for a Philco).