Doverham ,it certainly looks like a screw grip nut ,but not benefiting from the extension,as with the later Webley 600 and 700 range .Whether it is a Webley action or not I could not say from the picture . There were a couple variations on the theme ,I recall a Danial Frasier that had what looked like a grip nut but was in fact a boss that keyed into the action rather than a screw thread . The gun shown does not have a typical Webley top strap being longer than the norm for a Webley .