I don't think their are actual "proof" marks as they said.

The marks just indicate who worked on it or what shift it was worked on during or what quarter of the year it was made etc. (you might be "A" and I might be "G", "9" might mean September, etc.). This is so they can keep track of who did what and when if there are problems down the road.

"Proof" marks (as in acceptance after being fired with an over pressure round) are usually on European or military guns - not commercial pieces.