they are factory proof marks, there stamped after they fire proof loads.
Model 12's are still going strong but there are some things that need to be checked to see what condition the gun is in, how much wear and if the locking surface is starting to roll.
Bolt lock-up, timeing of the lock-up to firing, barrel to receiver fit. It could have been carried more than it was shot or it could be worn out and in need of rebuilding.
If you have any question let me know or do a search for m-12 issues and repairs.