[Arran]

I think that the schematic is just giving a literal DC resistance measurement value for the output transformer primary, such as one half of the primary is would on top of the other in a different layer. I haven't heard of any issues regarding replacing an audio output transformer in any sets with the screen grid inversion setup, they just used a replacement that matched what the tube types required. In terms of why it failed it could be corrosion, or it could be that the set was played with leaky coupling capacitors, C-206 being leaky could also cause a failure.
Regards
Arran