As the back plate being about 6 mm in thickness, with no sign on the inside of the turret plate of the focus tube .You would look at a very fine thread that would have to stay in place with no grub screw or locking ring that would have to resist the turning force when changing the turret eye position.
With all the above said maybe this modle was never meant to have a star diagonal or straight through eyepiece.
I still believe that I haven't fell on the the right answer, as an ex engineer these were the sort of problems I liked and always found a solution to.
Well one more strop down next week, will let you know what I find.
Can some one tell me how to add photos, I was unsuccessful last time I tried
As always many thanks for all your help.