[old_tv_nut]

The HV rectifier is screwed on the chassis, not the rectifier tube itself. There could be some kind of spring clip in the socket cup that holds the tube and has to be pressed down to release the tube.

See if all that black around the cup wipes off, or if the cup is deteriorated (carbonized).

I can't identify a focus block in your pictures. If it has one, I would expect it to be wired to the high voltage rectifier and look like a brick of plastic.

Not familiar with this chassis, so take anything I say with a grain of salt. Others here can probably get much more specific.