If that's the case, it would be a shame to just part it out without attempting a repair. I hope someone can answer your question about what was used to braze or solder this rib extension, but I've never seen that specific info anywhere.
I have read that it doesn't take much to hold a shotgun breech closed. Considering the construction of the rib extension of my Flues, I tend to believe that. If you cannot find the specific braze or solder used, I think I might just try some lower temp silver solder, or even Brownells Hi-Force 44. The regular Hi-Force 44 has a tensile strength of 28,000 psi and flows at 475 degrees F. while their Hi-Temp Hi-Force 44 has a tensile strength of 38,000 psi and flows at around 640 degrees F. I'd wire and nail the breech area together to make sure the joints that are sound remain that way. Maybe fill the barrels with water up to the midpoint of the chambers while held vertically to keep the rib joints well below a dangerous temperature. Braze the rib extension back on, and test fire it with the gun tied to an old tire and a long string on the triggers. If it works out... great. If it comes apart, try again with another silver solder... or go with plan B and then part it out.
