Alf,
As stated in the thread from the other forum, the "reject mark" is very common on P-38s – particularly the later “cyq” marked pistols made by Spreewerk. This is the first time that I have seen a photo of the mark on a Luger - but I am no expert on either pistol.
My understanding is that the stamp shows that the part failed the first inspection, but was accepted for use after the flaw was corrected. I do not know if the star burst stamp was applied to show the initial reject, or to show the final acceptance after the part was reworked.
For an example here is a photo of the mark on the frame of my P-38. For more discussions on the topic go to the P-38 Forum at
http://forums.p38forum.com/ and do a forum search on the term “reject mark”.
Mike