It is a professionally installed safety catch but sadly only has on or off positions as opposed to safe but operate the bolt position available on most Mausers.
It has been adjusted or made to suit a scope mount, the mount we dont think is original
Here it is uncocked and not applied;
Here it is cocked and safety applied, the only way it will work!
There is almost no clearance between the objective lens and the top of the safety though!
It still works though. One of its first outings in the wood last summer;