Certainly a scope for a .350 Rigby rifle with the Rigby mounting system , but not fitting the "Blixen" Rigby.

The Blixen has the typical Suhl claw mounting system.

For me a strong indication that the mounting must have been later added on the continent.

BTW what is named Rigby mounting system is not uncommon on pre WW1 commercial Oberndorf sporters that have never seen Great Britain.

Sauer und Sohn / Suhl used to build a nearly identical mounting system on their pre WW1 Mausers:

The hind square mortise identical to the "Rigby" but the front base used
a square stud instead of the round "Rigby". Otherwise the mechanisme was identical