John, it certainly would be nice to see the outer surface of the missing piece. Somehow it let go along the braze joint, so either it was a bad braze or there was corrosion - at least that makes some sense to me given what we have seen so far. I think the dark spot on the break would be confirmed on the other piece as well. I think this gun was doomed at birth, but we may never know.
That does appear to be where the initial fracture took place. Similar in that respect to the gun Bell subjected to gross overpressure (load in excess of 30,000 psi, if my memory serves). It blew out from the void between the barrels where the extractor rod rides. An obvious "weak point"--but not very weak in that gun (a vintage Parker), given that it took that much pressure to result in a catastrophic failure.
The only failure I've had a chance to look at (a few minutes after it happened) similarly started at a weak point. The gun in question was a Fox, which has slender rods (or guide pins?) on the outside of the extractor, riding in channels just below each barrel. A friend, looking at the gun with its blown right chamber, pointed to the void where the pin rides, thinking it was a flaw in the metal. And it very clearly blew from the outer edge of the barrel inwards, toward the rib, rather than from between the barrels out. Just part of that particular gun's design, but also an area which is weaker, thus vulnerable to failure if there is significant overpressure in the breech area.
