That is a firing proof - called a Beschutz. Unless late war it should be on both rec & bbl on left side.
Are there any markins/symbols on the right side of the rec above the stock?
What markings are on the bbl between the rear sight and receiver? What markings on the right side of the rear sight sleve? On the sight parts?
If infact there are no other markings then the Beschutz & ser nr, I very much doubt it is a WW 2 bringback! It would Not be a rifle assembled from parts either because a parts rec would have had the mfgr proof on the right side and no Beschutz or ser nr.
Sarge |