If, during the rework, the breech block - a major functioning part - was replaced, then a proof mark was likely required just as it would be on the new barrel. I'm fairly certain that the proof mark is a C/N.

None of the OP's pictures show the breech block serial number but I suspect that it's been replaced. Even if not, it's still reasonably logical that the breech block would be proof marked. Reworks after WW1 are a wonderland of unusual variations and markings.

If this Erfurt is like one I have the number on the breech block is on the left side, toward the rear and not visible with the toggle in the barrel extension. The extractor is numbered, for whatever that's worth. Only Yanik213 can tell us if the block is numbered. The pistol, having been military, has military proof house firing proofs. For non military use it would have been proofed again by the civil proof house, which ever one that was for this gun.

I would like to say the old military firing proofs would not do, but just as soon as I do there'll be all kinds of folks with post war military reworks, Germany marked, that have no civil proofs. Point being one can make no flat statements, exceptions will pop up.

I would very much like to know who reworked it and where it was proofed.

By the way, from what can be seen of the magazine in the one picture, it looks very nice, also.

Jack

see Dwights comments

Thanks Ed, for sending me back to the beginning of this thread. Dwight Gruber's assertion made me research a little further and answer one of my own questions. The forth proof on the frame rail is there as per the 1891 Proof Law.

Jack