Alright, I threw all my retard strength into it and finally managed to get the gorram knob off, and did a proper inspection of the thing.
The dial cord is indeed broken, and I think most of it is gone. It appears to be two separate pieces, there's a length of thin wire with loops on the ends, and that's still intact, but then there was also a nylon(?) string, of which I only have a couple inches. Additionally, there may be some other piece missing, since I can't otherwise figure how the tuning indicator would attach to the string. I will
definitely need a stringing diagram to figure out what's going on.
I also made another less-than-pleasant discovery... one of the output transformers has been replaced with a (much smaller) Triad 'universal' one, and the circuit may have also been modified to accommodate it. There are a couple unpopulated spots on the board for resistors and capacitors, and the soldering there doesn't look factory original. So that sucks, looks like I'll need a full schematic too.
I don't know what the chances are of finding another original OPT, but I guess worst case I could just replace
both of them with a matching pair. There seems to be a fair supply of Telefunken SE EL84/EL95 ones on eBay that should work well with the ELL80 and wouldn't look too terribly out of place, or Edcor probably has something functionally suitable as well.