[Arran]

Check the resistance of the speaker field coil, maybe someone swapped out the speaker for another out of an AC set like a Philco 60? If so that would explain why the output transformer does not have a center tap. Either that or someone did not understand that the set has a push-pull output stage, or intended to convert it to using a single 42 tube and replaced the original center tapped unit.
If someone added power resistors to the filament string clearly someone was messing around with this set before, and lord only knows what they had in mind. Unfortunately when people do things like this they seldom make a diagram, and we can't read theirto see what they were thinking of at the time. This is why when I run into such things I undo it all and try to return the set to the way it left the factory rather then trying to make it work with someone else's kludging in there.
Series connected filament strings are really quite efficient, the current consumption is considerable lower then if they were connected in parallel. They is one improvement that could be made over the original design though, and that would be to add a current limiting thermistor in series with the tube heaters.
Regards
Arran