On the Rigby, locking is on the front base and the rear square bridge is wedge mortised while on the current Mauser claw system the transverse lock in build on the rear flat top with a claw base on the receiver front ring or on a base soldered on the barrel.
Indeed, the Rigby "flat" is a provision to ease the fitting of the front scope mount base.
If you study the first scope mounting systems made at Oberndorf you will notice that the kind of arrangement found on Rigby was first used by Mauser (Mauser delivered the complete barreled actions in the white to the English firm that did the stocking, sighting and regulating)
I have seen at least four pre-1905 commercial rifles scoped that way, with the locking by a small lever on the front base.

DORLEAC
www.dorleac-dorleac.com