Patrick,more than likely the rib was soft soldered. soft solder will flow between 250>275 degrees.Hot bluing salts are usually at 275>280 degrees which
would explain why the rib came loose.If the barrels have separated they will have to be fixtures and re-regulated and then re soldered with soft solder.
silver solder will require a 475 degree temp and will affect the heat treat of the barrels, soft solder @ 275 will not.
If only the rib needs to be re attached,I would use a very sharp 1/4″ wide chisel to remove old solder and smooth the surface under the rib stanchions then clean those surfaces with acetone and apply red loctite (#271) and gently secure the rib in position with safety wire, iron wire or cable ties and allow the adhesive to cure overnight.I have repaired several old shotguns in this manner and they are all still in the field. Good luck and keep us posted…
