Huvius:
Thanks for the link: I hadn't seen that one. Much appreciated.
I have been collecting web-pics of these mounts since taking delivery of the .275, and that is the first of a good half-dozen that is clearly a post-manufacture added rear mount (by Rigby no doubt). All the others I have seen appear to have been manufactured as single square bridge actions from scratch.

The fact that Rigby also appear to have added such a mount to a standard Mauser 98 action is very encouraging!

Forum member jcorry has posted photos of a lovely .350 Magnum with the rear square-bridge recessed for the detachable mount. Click HERE to view the thread with photos.
Unfortunately jcorry hasn't posted since early 2014.

Pjaln has one too I believe, maybe a 400/350, and I have sent a PM requesting dimensions, with fingers crossed!
Meanwhile, if anyone else here has such a rifle (or won Lot 2277 at the JJ auction!) I would love to get accurate measurements for the length, width and depth of the square hole, and internal angles of the mating surfaces.

A pic (and dimensions) for the scope-ring foot would be mighty helpful as well!

Thanks all