50C5 is not really oddball, that with the 35W4 was used in a million radios. We just don't see them in guitar amps much.
There were two transformers in this amp. One was the output transformer, which would be wired to pin 7 on the 50C5 and the other end to pin 7 of the 35W4. Check your wires. It was not rare to find the OT mounted directly on the speaker frame. Then a couple wires from the chassis run to that.
The other transformer is the 12v transformer for the 12AU6 heater. The secondary is wired to the 12AU6 heater pins. it is not a straight 120/12v transformer, as its primary is in series with the two other tube heaters plus a resistor. In fact we can calculate what it is.
Heater current 0.15A (150ma). SO nominal 50v and 35v dropped across the tubes, and 100 ohms drops 15v. SO 100v drops across everything but that transformer primary. leaves 20v I never saw a 20/12v transformer but it is possible.
SO you determine which transformer in the schematic is the one you see.
I googled TF103 and find it is a replacement Champ type OT from Triode ELectronics, (and maybe from others too) I am sure that would work just fine here.