BFJ,
Yes that's true, but a wedge, fitted between that angled cut on the barrel extension and the top of the action frame would create a similar result.
The wedge or bolt can be fitted in a machined recess in the upper action frame forward of the top lever, and is an extension of the top lever and rotated into position as the gun is closed and the top lever returns to it's closed position.
The Webley PHV-1 works like that. It too has that angled cut on the barrel extension but it is NO screw grip because there is no thread or screw.
The other Webley boxlock, the A&WC, is a vastly different third bite system where the angled barrel cut is engaged DIRECTLY by the top lever SPINDLE which has a thread. This is a true screw grip third bite.