I don't know what could have happened but your analysis seems sound so far; and I'm very thankful that Roy didn't suffer a severe hand injury! About 50 years ago (hard to believe that long ago now), my brother in law managed to blow the barrels in my pristine 1919 vintage FWE 12-bore Field Grade. In that instance he was horsing around and unknowingly managed to obstruct the left tube about 8" from the breech. The next shot vaporized the flesh between his thumb and index finger and left his thumb pointing towards his nose. The barrel split much like in the photo above, but with an obvious bulge at the obstruction. Barrel ribs did not separate but the explosion bent the tubes sharply left and downward. Forend wood was splintered, but the iron itself was not damaged. Wish I'd taken pics, but my focus at the time was getting my B-I-L to the emergency room. Kept the barrels for years before sending them to Buck Hamlin, and I've no idea what may have become of them since. The only good thing for my B-I-L was that his injury kept him out of Vietnam.