The action would have been color case hardened. I will say IMHO Bakers had some of the prettiest colors out there. The rings are kind of odd to me. I wonder if the barrels were honed and that is what you are seeing? I agree, measure the bores and the barrel thickness, just to know what may have caused that.

Oh no! Was that bridge made from Damascus or composite steel? Regardless, it will always come down to several factors, when steel comes to failure.