[Arran]

It's a code 121.

The schematic on nostalgia air also shows a 6J8 but the internals show a pentigrid convertor like a 6A8 not a triode-heptode. The schematic in my 1939 Philco book also shows it marked as a 6J8 but shows a pentigrid convertor like a 6A8. The 39-116 definately uses a 6A8 but since the 39-55 is basically a budget minded mystery control set with only the broadcast band I don't know why they would use a different tube. The only other Philco models that show a 6J8 in the tube complement are in the Tropic series, and those show a triode and a heptode section in the tube.
Regards
Arran