These were indeed uused on the old Mk1 Centurions and later, the BESA mounted Mk3 Cents. Hence the silver paint job, that distinctly shows the blood as you knock your head against it in the confines of the turret. Much easier to colimate to the machine gun or Polsten cannon than the No42 or 53 with their eccentric ring mounts
Changing the graticle diaphragm - or modifying the old optiglass cross-hair grat is simple enough - if you know what you're doing
A bit more difficult is the removal of the old No71 engraved markings. Of the dozen or so I have converted, I simply bead blasted the area and lead loaded those markings, then over engraved with No32 Mk whatever it was originally.
If you buy it, take that into account.