I don't know, but just from looking at the picture I am thinking that a .357 magnum even a proof load, just doesn't have the power to do that kind of damage, not even with the barrel welded shut. The cylinder would have had to fail in several places for it to be broken into two or more pieces, and then the frame would have also had to fail at a crucial point. Smith and Wesson are fairly well known for making a fairly robust revolver, and this could only be the result of a perfect storm of manufacturing defects by not only Smith and Wesson, but the owner, and the ammo manufacturer.
ETA: It also looks like there is little to no deformation of the forcing cone, if there was enough pressure to do all of this damage, you would certainly expect to see the forcing cone sustain some damage. Also, wouldn't the bullet of the round in the chambers not lined up with the barrel be severely deformed from having been cooked off (if they were destroyed by the catastrophic failure, with all of the hot gasses flying around the powder in the other chambers would have been ignited causing them to at least have scorch marks on them).