In the past they were all silver soldered. Quite a difficult task on a
side by side double as the barrels were also silver soldered together
and trying to add a rib was likely to cause the barrels to come
unstuck. Back when I was involved in gunsmithing George Hoenig of
Boise Idaho was the only gunsmith in the U.S. skilled enough to do that
sort of job. He built multibarrel shotgun/rifles from scratch. Now he
builds $30,000.00 shotguns of his own design.
With the current technological improvements in adhesives you might get
away with some sort of epoxy especially with a nylon or lexan rib. If
you suceed in your efforts you might well have a product you could
market if the price wern't too high. You don't have much to lose by
trying on inexpensive single bbl guns so have fun with it. Good luck.
73 Gary