My guess, and it is nothing more than a guess with those pics and the brief story regarding the barrels, is that the replacement barrels were not fit up correctly. If not enough materieal was removed from either the hook or the rear portion of the lumps the resulting cracks could occur from attempting to force the barrels into place. As Mark suggested, look for the source of the problem and fix it before the cracks at the rear of the slot.
The cracks can be welded and the engraving can be recut, blending the finish will be a trick but can likely be done. The area of the repair is right where CC would be most prone to wear, take that into account as the finish is blended out.
Good luck and keep us up to date on this one, it is a worthy and interesting gun.
