2. The color and uniformity of the barrels suggests previous re-blueing with hot caustic salts.
3. The extensive rusting suggests that the silver solder between barrels and ribs dissolved during the blueing.
4. The long split without a bulge suggests inadequate wall thickness; possibly from honing when the barrels were blued. Measurement of the wall thicknesses of the barrel remains would confirm.
SO 1st the thin left barrel burst, and 2nd the non-fused barrels and ribs separated.
Feel free to contribute other thoughts. I most certainly could be mistaken.