All the old ribs on double barrels were soft soldered on and so were some early vent ribs. This is why they can't be tank blued, as the bluing dissolves the solder. (The barrels on old doubles were soldered together as well, so tank bluing them resulted in four pieces.)
As noted above, most of the later ones were silver soldered, but some of the ones seen recently seem to be copper brazed.
Jim