[Arran]

In my opinion the ARBE III is complete overkill for a 1940s era farm battery set, those only have two voltages required, 1.5 volts for the "A", and 90 volts for the "B", I don't think I have seen one that required a "C" battery. With 1930s farm sets it's a little more complicated, those usually require at least three voltages, 2 volts for the "A" (three if connected in Air Cell mode", 90 volts for "B+" and 135 volts for "B++". Some require a "C" battery, which is really for bias in the power output stage, most of those are -4.5 volts, you can build a supply for that, but since there is no load on the bias battery you can just use actual batteries, and they will last for their shelf life.
1920s battery radios are paradoxically simple, which is why they require a more complex power supply, for whatever reason they had not clued into the idea that they could build a voltage divider into the radio rather then using three or more B+ voltages. But then again the 1920s was deluged by constant patent wars, where the ones making the big profits were the lawyers, so a lot of engineering decisions were made on the basing of evading royalty payments.
Regards
Arran