I have changed my posting because a detail bothers me , the thumb hole.
As I have already written , I have tried to put the bolt body from a standard Mauser action in the receiver from a Mauser Magnum action and had the same situation as on the pictures.
If it's a Mauser Magnum action , it's not a new action because it does not have a thumb hole. However , there was a pre-war Mauser production of single square actions so called magnum ( or not magnum ? ) systems with thumb holes. They made rifles caliber 416 Rigby with it. As I said , I would look in all cases for a Mauser Magnum bolt body.
Good luck with the search of the matching piece.