[Chris Cusick]

Aha ! I see what you mean. If you look at the wiring diagram, that cap is going to pin 1 of the 6H6, which is just a ground for the metal tube shell. So yes physically it goes there but it is not actually inter-acting with the 6H6 in any way, which explains the difference between the 2 diagrams. That's just a convenient place to use for grounding it. The RCA 810K1 was one of the pioneers for metal tubes which used this arrangement for built in shielding. Not sure that they were really that great but hey it's part of the history of your set.