[Arran]

An easier way to explain it is that a 5U4 has the same pinout as a 5Y3, and I believe that a 5X4 may have the same pinout as a 5Y4, both the 5U4 and 5X4 are successor tubes to the 5Z3, which is pretty much a heavy duty type #80.
Getting back to this 37-660, I would go by the diagram inside the cabinet, I can't read the code number on the blue label but I'm guessing that this may explain why the version linked too was a mirror image. The set linked to also uses a 5Y4 for a rectifier, why I don't know, hopefully the rest of the power supply matches the specs of that tube rather then the 5X4 or it won't live very long.
Regards
Arran