No question this gun is older than 1924. It was made for 2 3/4" length shells (70MM) but it wasn't made for modern 2 3/4" shell pressures, especially SAAMI standard 12ga shells as currently manufactured. If it were mine I would want to stay below 1 1/8oz and 9000psi as you suggested. There are plenty of 12ga loading formulae which will give you that and provide full performance. If you don't reload, might check out some of the vintage loads available from RST and Polywad. I use 2 1/2" loads from RST in a gun at least as old as yours.