12C Indicates 12 ga chambers at time of proof.If 10 ga Shells fit chamber and rims two scenarios. A. opened up to 10ga later or
B.Mis-marked at proof. Check firing pin centers. Std 12 would be 1.125" 10ga 1.187" ( approx) Photo of end of chambers/breeches would help.


Hugh Lomas,
H.G.Lomas Gunmakers Inc.
920 876 3745