I'll vote with Jani on 11,2x72. The 10,9 mm indicates bore, not groove diameter to the nearest 1/10 mm, 72 showing chamber length in mm and a St.m.G. = Stahl-mantel Geschoss (steel jacketed bullet) of 26 grams (401 grains) all perfectly describe the 11,2x72 Schuler.

There's a thread on the Accurate Reloading boards that talks a bit about the round ==> here <==