Hi Mauro,
Pulled the pistol apart and found no other parts with engraved parts of the serial number, just the rear toggle as shown before.
Some unfinished pistols from Mauser production were sold in their original assembly boxes, as shown in the image below. The best explanation I can come up with is that several small parts were worked on outside the assembly boxes and were marked to ensure they would be mated back with the right gun after the work was done on them. Why this meant that some parts were numbered or marked and others weren't is beyond me. It seems to prove that Mauser Parabellum production wasn't exactly a full time job and that projects were halted, stashed, worked on, stashed again, etc... over a large time span.
