In the absence of any other information I would hazard a guess that a pinhole has occurred in one of the barrels in the space between the top and bottom ribs and escaping gas has blown out an escape split through the bottom rib, hence the piece of rib standing proud and the pattern of rust? blown out onto the barrel and rib surfaces. I think that blown rust? pattern is a give away as to what has occurred.
Can you see any sign of a pin hole/s inside the barrels, maybe hard to see if barrels are worn and dirty?

Ah sorry was looking at the protuberance on the bottom rib, not the one on the side of the barrel.