Metchosin......
I found a CD reissue of a Lonnie Donegan recording titled, "King of Skiffle." There were no liner notes, but "Rock Island Line" was on there along with "Wabash Cannonball," "Michael, Row The Boat Ashore," "Don't Rock Me Daddy-O,"...19 cuts in all, for the low, low price of $6.00. Reckon this recording would have the Rory riff you were talking about?
Neil