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That's really cool that it's the same chassis as RCSs period documentation pictured!
It's possible if that set was being used during WWII (maybe with development of bomber cameras or as a monitor at an NBC TV station, or in a military hospital in the US) and the 117Z6 was a wartime tube shortage workaround to the correct tube being unavailable. IIRC the 117Z6 was available prewar and used for the AC supply in Battery/AC portable radios...I have to wonder if it needed it's own isolation transformer to avoid heater-cathode shorts.
RCA was factory updating their 39-40 models as late as 48 and probably supporting them longer...They kept prewar sets around for that and may have kept this around for some purpose that required it to work.
The 12AP4 can be tested on a modern tube tester. I checked mine (after dealing with bad soldering on the heater pins) on my B&K 466 using 10BP4 settings for everything but the heater and was able to get a decent idea of emissions.
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