From Gough Thomas (Shotguns and Cartridges):
An outstanding case in point has recently come to my notice. It concerned a one-gun man who shot game successfully but was no good at all at trapshooting, with its more deliberate technique. But when he fitted a supplementary, superelevated rib to his gun for trapshooting only, he became the star performer at a well known Continental gun club.
If anyone has pics to share of "supplementary, superelevated ribs" they have mounted on their sxs, I am sure they will be favorably commented upon.