any Gew98 marked 1920 and if see if any are Circle S marked.
I was reading the the Still books and studying the circle s luger photos and had this same exact thought. Still actually has a surprising number of circle S marked lugers photographed.
If it helps, Still says in "Weimar and Early Nazi Lugers" p.36 "An Oct. 19. 1922 decree by the Prussian Ministry of the Interior ordered the major repair of the K98 {should probably be G98}, MG08, MP and P.08 to be undertaken at the Simson and Co. Suhl factory. (Wotka, 1993)."
Same book, p. 25 regarding the 1920 stamp, "Imperial Lugers intended for Weimar, military or police use were reworked by a number of firms including Simson and DWM, and by miltary and police armories. ... Most bear the 1920 Reichswehr stamp and were routed to Army, Navy or police use during the Weimar period with the majority going to the police."