The Square within a circle was originally the symbol of the 20th Arsenal in the City of Chungking in Sichuan (Szechwan) Province which was established in 1938 with resources from the Chungking and Nanking ammunition factories. The 20th Arsenal originally was the Sichuan Coin Bureau (a mint), established in 1929 and was later converted to an arsenal. Note that the symbol resembles a Chinese Cash coin of the period.
The 20th Arsenal was the principle cartridge manufacture in China during WWII and continued to produce ammunition until the fall of the Nationalist Government in 1949.
The Communist Chinese Government continued to use the symbol until at least 1951.
Ref.: following articles
Guide to Chinese Headstamps !880-1950 by Lewis Curtis, Bin Shin & Bill Wooden
Chinese Cartridge Headstamps by Tzong Bin Shih