CommandCar,

The proof marks denote regulation with the 750 grain bullet and Cordite, its also stamped with an 'S&B' which I believe, stands for Shot & Ball, also there is a '12C' within a diamond and the 1 1/8oz shot weight proof mark too. I'm hoping to seal the deal on this 12ga hammer by next monday. Will post pics here if I do! Fingers crossed!