This is a chinese copy of the SIG Bergmann 1920 itself a licensed version of the Bergmann MP 18.1
As I found out there in Shandong in 1995, the workshop was established by Vollmer and produced submachineguns for the Public Safety.
The SIG Bergmann was in .30 Lüger (7,65 Parabellum), the "Tsing Tao" SMG was chambered in .30 Mauser (7,63 Mauser) and the magazine well was vertical unlike the other SMGs.