I am inclined to think that the other holes in your rear bridge are for the original Rigby mount such as on this rifle:
http://jamesdjulia.com/item/2277-369/

It is really a simple base as it has just a square hole in it to accept the square hook on the rear ring. These are tilt back to remove and the front mount is where it locks down - which you already have of course.

Lovely gun. I also prefer the full pistol grip stocking which seems to be rare on the .275s.