I would not at all be surprised if the S B 7.65 headstamped cartridges with Sinoxid primer and red primer seals were not made by Geco (or RWS if before the working agreement of the c.1927 when Geco made all the commercial pistol ammunition with both GECO and RWS headstamps). My reason for this comes from both cartridge characteristics and the headstamping itself, not even to mention the “O” marked Sinoxid primer.
I have two variations of the Schönebeck-made “SB” headstamped cartridges ( /SB//S/ )
one having a CNCS FMJ RN Bullet and the other a CN FMJ RN bullet. The two cartridges, other than the difference in bullet jacket material, as essentially identical.
I also have the S B 7,65. headstamped round. I compared it to a typical RWS headstamped round and both are identical to each other other than the factory designater. Especially telling is the dot after the “7.65.” on the SB round, as well as the printing font of the caliber marking. The dots in various numbers and in positions before and after the factory designater and before and after the caliber marking, possibly a machine identification marking or even a date code, is typical of RWS/GECO 7.65 mm cartridges from about the 1930s. These dots can be found in any of the positions mentioned above, or all of them - in short, a single dot, two dots, three dots or dots in all four places. Both have identical extractor grooves and extractor groove bevels, which differ from those of the /SB//S/ rounds, which certainly were made by Sellier & Bellot Schönebeck. Both have the red primer seal and case-mouth seal common to the RWS/GECO cartridges. Both have the same exact ogive to the bullets, both of which are CN RN FMJ.
Since the S&B Schöbeck was taken over in 1933 by Rheinmetall and Dynamit A.-G., and there had been an amalgamation of RWS with Dynamit A.-G. in 1931, It seems quite plausible that either GECO of RWS (under the 1927 working agreement) could have made these cartridges, perhaps as a sub-contract from D.A.G. for their Sellier & Bellot branch.
First and foremost, though, it goes back to the cartridge characteristics of the S B 7.65. round, which are purely GECO/RWS.
Just my thoughts on this.