The gun pictured is from an email received by Oxford and forwarded to me. Ox is attempting to find the details. But I am betting that it was a double load of unique or bullseye. Doesn't look to me like a squib jam as there is no barrel bulge.
I saw one picture and one real gun opened up this way and looked about exactly that way with three chambers split and the backstrap bent/broken.The two I know of,they were using a light charge of BP in blanks to break balloons.They found they could load cornmeal over powder to fill the case and it helped break balloons.I make no assesment on what happened here. ,,,sam.
I read an article in a magazine (SHOOT I believe) devoted to cowboy shooting where two guys managed to blow up 2 single action revolvers. They were using a Dillon 550 and noted that occasionally that a bulletless case would fall into the loaded round bin. They assumed that they had failed to place a bullet in the case mouth.
It turns out that the original bullet had jammed up inside the bullet seating die and on the next round the added bullet was pushed deep into the case on the second try with the first (the lost one) bullet seated on top,producing a normal appearing round..
It was a miracle that these guys managed to blow up 2 revolvers without hurting themselves.
They were also quite frank in writing their article and didn't seem the least embarrassed.
DocAitch