Joop helped me understand many things about Mauser and their production processes. I met him in Oberndorf and will always treasure his friendship.
He looked over the Mauser Luger I shared above, and confirmed that his opinion was it was a factory numbering error and overstamp. A number of other characteristics of the pistol confirmed it. It's always important to evaluate such things in hand, and look at the pistol as a whole.
I was also initially concerned about it when I acquired it, but he helped me resolve the issues.
The same thing proceeded with evaluation of my Mauser logo toggle pistol, 715g.
Hi, I recently acquired this military proofed E/135 9mm Mauser luger, with all matching numbers and a military proofed FXO magazine without any serial number. It has a forward toggle that is marked with a Mauser Banner. From the inspection/sub-contractor mark the toggle was probably...
ALTERNATIVE EXPLANATION FOR A MILITARY 42 DATED BANNER LUGER IN THE “g” SUFFIX BLOCK. Joop posted: “Now about your 42 banner serial number xxxxg. It is a Luger pistol that was inspected and accepted by the German Army inspector 135 (code for Major Rosenhagen). The 42 date on the chamber, and the...
Note that this pistol is pristine, appearing as it would new from the factory. It has signs of factory grade bluing in places where you would not expect it, and so may have been finished to make all parts from different batches appear more uniform for commercial sales.
All below is speculation without any documentation.
I expect that Mauser accommodated all customers as best as possible, with the government contract their major customer during the war years. Commercial sales would not require the use of toggles with concealment codes on them, so you'd expect to see the Mauser Banner barrel, and batch production of the other parts would probably not be interrupted for special runs of parts not inspected for the military contracts.
As it became necessary to fulfil small commercial contracts, batches of finished parts would be pulled from existing stocks to complete the required number of pistols. We have parts assembly process diagrams so those processes could probably be researched. Final finishing and assembly sequences could be determined.
If parts were scarce, and that could be especially true for major parts like the receiver or frame, you would expect them to be sourced from anything acceptable still available in the factory. That could include older leftover stock and might account for the use of earlier year receivers on some of the commercial contracts like the Swedish contracts. Nothing implies that these were rejected parts.
Without documentation, we'll have nothing more than evaluation from people like Joop, members of this forum and our own collector opinions.