Mysterious Dots

[RP]

I recently purchased a 1936 S/42--all matching/original, with 5 dots(4 dots outlining a box shape with the 5th dot in the middle of the four). Looks very similar to Thor's pic, and is in the same right/rear frame recess area. These dots, although small in diameter are still much larger than any Rockwell hardness tests that I am familiar with.