Close examination (and a bit of image processing)reveals that there may be a witness mark across the barrel and receiver.
Jussi, would it be possible for you to take the gun apart and photograph the bottom of the receiver for us? It may be a tantalizing addition to the witness mark and the Luger assembly mystery.
Bob (Rockin'WR) has a good point about workers stamps, if you find any under the receiver or in the receiver well (or anyplace else) it would be good to know. This is a very unique and interesting Luger you have.
And it does seem that Steamed TACFoley does, indeed, have X-ray vision!
--Dwight