Dwight Gruber is the expert on the circled-S stamp. Four years ago he published this interim report:
http://forum.lugerforum.com/showthread.php?t=16590 . I don't know if he has any more recent insights but this report indicates some Erfurts have the marking. I was thinking that there might be a correlation unique to the Lugers of the Mannheim Schupo but my review of information in my database collected since I published HWIS threw a wet blanket on that speculation. As far as I know, there is no good idea as to what the mark means.