I have a set of 20 gauge Ithaca Flues barrels that had the rib extension come off. As I recall, it isn't any longer than what you have there on your NID. It has a small alignment pin that goes into a small hole. It looks like it was joined with solder, but a lot of silver brazing solder has the same coloration as regular tin-lead solder. There is a considerable difference in tensile strength in silver solder versus regular tin-lead solders.

Voting for anti-gun Democrats is dumber than giving treats to a dog that shits on a Persian Rug