Is it a later Field with a Single Sighting Plane rib?
May we assume you have already measured the wall thickness around the edges of the burst?
It does not look like an obstructional burst with a ring bulge, and if the top rib is not lifted likely was not; the bottom rib obviously separated
nor does it look like the usual thin wall split
Circumferential burst at mid-barrel with jagged edges (abrupt cleavage with little plastic deformation) suggests a barrel defect.
At the breech, very high pressure.
Could there have been a previous dent or bulge that was inexpertly repaired?
This would seems to be a good opportunity for Frank to cut and paste from a textbook of metallurgy and help us understand how barrel steel gets "old and tired"