I have not had one off, but I have weighed my 20 gauge Ithaca SKB 100 barrels and they are very light, even compared to my english guns. Based on that I suspect that the top rib on mine, at least, is hollow. But that is just my guess.
I also have a 1970s vintage Ithaca SKB 200E 20b, which I believe it has a small middle bead, and I think that recall that I took that out years ago and probed with a wire and concluded that the rib on that one was hollow. But I might be thinking of a different gun, I'll look at it tonite and confirm.