Is this thing radio or TV related? (see the attached photo)

Looks radio related or perhaps a rectifer or other power supply item. What did the eBay listing say? I recognize the file name as an eBay image (S-1600) that the date was added. I bet the seller even has no idea but probably thinks it is worth a lot of money.

Big vacuum tubes were most commonly for RF amplification if I remember correctly. Therefore my guess is a big triode.

Or a tetrode (common in RF medium power applications).
Does not have to be radio or TV, it could be as well for an industrial use (induction heating, ultrasonic generator,...).

Interesting they used bolts/nuts for inner assembly, without anything apparent locking, I would be afraid it gets loseafter a few heat cycles...

Yep this is on Ebay. The seller didn't know what it was either.