It is a gas vent,in case of brass case head rupture.
There is a corresponding vent in the bolt body,to vent gas out of the bolt.
One of the differences between a mauser 95 and a 98,the other being a third lug on the bolt body.
Definitely Factory.
This is the reason I sold all my mosin nagant rifles,no provision to handle escaping gas.

**********************
[the member formerly known as fluffy}