Heres a different thought-
If you look at the area I circled in your photoghraph, the milled out oval nearly goes out the back of the mount. This mount may not have passed inspection tolerances, therefore was never waffenamt as aproved. It is conceivable that this mount would slide far enough forward that the locking lever would be useless and the front end of the mount pressing hard against the stock making it an unstable platform. What you are seeing are the measures needed to adapt it for use. Cut off the front end relieving pressure on the stock and mount, then remove the locking lever because it either does not line up to work properly for fit or it never set well to begin with. Close-up pictures of the front end that was cut off would be interesting to see.
As for Gergs comments, they are true to a point. Those indicators are true but not the only thing. Many mounts do not show the milling marks, especially later war ones. It all depends on the miller and the depth of the tool.
Just a different perspective.