Geno,
Any engineer will tell you that the cut for the cartridge RIM in the breech ends of the barrels also makes them weak-but, every single gun has them. And, the only guns that fail, in that area, are being, or have been, abused. Conventional guns, or, Darnes.
You said it yourself. A double nitro load was suspected. However, I believe a bore obstruction was more likely, if, in fact, the crack is related to internal pressure-it appears to be closed from the outside, and cracking inward.
Admit it, Geno, there are a lot of old Darnes out there, the number 600,000 was out the door by WWII. If they were as bad as you make them out to be, we would see many, many more in damaged condition.
We don't.

Best,
Ted