The proof marks are on the outside of the chamber, on the left flat in the case of the commercial pistols, and the right for military.
The "N" in the well indicates a DWM frame, and the rear picture shows a later frame, post 1914 or so. The lug has been removed by grinding or similar method and is "flatter" than a frame that never had a lug.
The serial number on the front is likely original and confirms a mid to late 1920 commercial mfg luger.
JMHO.
The small retaining pin:
PM "lugerdoc" on this board, he may have an original pin- but there is nothing magic about it- any pin that is a "snug" or "easy drive" fit is fine.
Just use a larger diameter nail, or turn a pin.
