[Arran]

Russ;
  I think that the 3Q4 must have been another one of those transitional type tubes that was replaced due to electrical code issues, like a 50B5, such as if it were plugged into the wrong socket it would liven something up. I have a lot of 1940s and 50s farm battery sets, and have owned a number of three way portables over the years, I only have one that used a 3Q4. It's a Canadian Marconi model 260 from around 1946-47, and the 3Q4 isn't even in it, it's one of those weird three way portables that developed the voltage for the battery tube filaments by connecting them in series with the cathode of the 50B5 output tube (who's filament was in series with a 35W4 and a dropping resistor), and in AC mode the 3Q4 is switched out of circuit and not used.
Regards
Arran