The St.M.G. stands for Stahlmantel Geschoss. Translated that means steel jacketed bullet. I have an earlier J.P. Sauer on an Oberndorf action that has 2.5g GBR under the St.M.G. That stood for 2.5 grams GBR (flake rifle powder). I would expect yours is the charge weight of the powder used for proofing at the time.