I believe the screw grip is a Webley design, but I'm not sure. My pair of William Evans sidelocks have the Webley screw grip. Of course, the whole pair of guns could have been made by Webley for Evans. It is one fine pair of guns and the screw grip is a wonderful design.
It is indeed a Webley design, referred to in Crawford and Whatley's book as " . . . a type of rib extension patented by T.W. Webley in 1882." It continued to be used on Webley & Scott guns, most commonly the 400 series, after Webley & Scott merged.