From your latest pictures, I think it's possible to see that the serial number of the frame was removed at some point.
This front surface of the frame is normally completely flat.
Just below the level where the receiver rail rides in the frame, you can clearly see the edge (on both sides of the barrel) where the horizontal grinding starts. A part of the last digit may still be visible.
The barrel base also shows some tool marks and unusual contours, particularly visible near the witness mark...
Marc
__________________
 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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