Underlugs for wooden under ribs, are usually attached to thin, folded metal, lugs, that are soldered to the under side of the barrel. I have seen more of these on half stocked squirrel rifles than fowlers, and , these were usually dovetailed, instead of soldered. I am sure they were used on fowlers as well. this was a method of making, and using, an under rib when the machine tools, and the means of successfully completing a large sweat soldered joint weren't possible. Often fullstocks with broken forearms were made into half stocks by utilizing the existing barrel lugs, and replacing the forearm with a wooden rib.
Hungry Horse