That gun has undergone at least one reproof, when it picked up the "tons" marks. If it's now 2 3/4", that change apparently was made since reproof--which does indeed mean the gun's no longer in proof.
The value of the proofmarks is that it tells you how the gun came from the factory, and what "official" modifications have been made. But as eightbore says, that does not mean you do not verify with a bore and choke gauge, chamber gauge, etc--because it may well have been tampered with since original proof or reproof, as this one apparently has.
