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Old 4th January 2020
Lives for gear
Looked closer at your first photo. The 70.7v tap clearly shows this was a piece of public address equipment... AFAIK, that's the only application that uses "constant-voltage" distribution to multiple speakers. (Each speaker had a transformer with a 70.7 V primary, and a bunch of secondary taps marked in watts, so you could easily set how much power went to that room. No messing with impedance... just make sure your total secondary wattage didn't exceed the amp. There was also 25v constant voltage, which had the additional advantage of class II wiring.)

Then I went to an archive of RCA catalogs, and looked at the PA amps in an early 70s version. None of them looked like your box. RCA's front panels were very different.

RCA -did- have PA amps in the "SA" series (I think it stood for 'speech amplifier'). But never SAGG as far as I know. And as far as I could determine, all of them also had MI ordering/stock numbers somewhere on them.