Alvin,
That is probably a hardness test spot...in the area you circled in "white".
Similar to our Rockwell hardness test. Probably done before final sanding and bluing.
You see those on various guns from time to time...probably evidence of a random test sampling of metallurgy of parts on the factory floor as part of their QA/QC process.
The Swiss liked doing such a lot. I have two M1929 Bern Lugers that must have 6-8 of them, apiece, on each gun and another M1929 without any of the "dots". I would WAG that DWM and Mauser and maybe others tested parts from time to time as well...
The edge of the area you show in "red" will usually show a slight cut of the milling pass. You can feel this cut edge with your finger. The frame is blued and then the polished area is milled/cut to create the polished area.
Sometimes you can see/feel that "cut" along the green and blue areas as well.
That shows in the 2nd. photo I posted.
I think it just depends on how deep they set up for the milling cut/pass and if the gun was held truly square in the holding jig or not.
Here is a different view of the "cut" on the same gun in the 2nd. photo above...a M1900 DWM-Swiss...
