Hello Dave and welcome to the forum , well done for finding those old photos – there is some explanation within the photos.
If you combine that info with the information from Harvey as reference by Barry you will have a better idea.
Problem is the old grey matter and memory isn’t what it was and I need some time to think it through properly, but for starters……
The metal Hymer extension fits onto the short original lock and is held by a grub screw.
i didn’t know for sure if it screwed onto the heavy thread (likely) or just fitted snugly over and held in place by the grub screw – and as you note, nobody else knew or replied.
The grub screw was corroded in- so rather than drill it out and try to remove the Hymer extension piece one way or another, I played safe and used the original body and just fitted the new handle to it.
My problem was that the handle would not turn because the body had seized in the black boss. This was caused by the white spacer piece breaking allowing damp and muck in. The spacer holds the plastic trim panel away from the boot. It is just plumbing waste pipe and can be pieced back together and fitted inside a larger 32mm waste pipe.
I cleaned up the corroded surface of the Hymer extension piece, greased it and put it back together using the old body boss ( black) and fitted the new handle to the old unit.
I removed the new lock barrel from the new handle and fitted the original lock barrel to keep the keys the same. It is still working fine.
Hope this helps, I’ll try to recollect my thoughts and also research the Exsis Book if I get time ( in the middle of bathroom project at the moment)
Fred