There might be a fair chance that something was machined out of round, the groove for the rim of the shell does not look uniform. Any old pictures of the chambering setup during manufacture? If the rim recess was cut with the chamber reamer, some other secondary machining very well may have been done in there.

Still, I'd think that if there're signs of pitting in the chamber towards the rib, one might wonder if the hoop strength in the area is affected, knowing it starts off relatively thinner.