I have an Evans boxlock, #2 of a pair, which was made by Webley & Scott based on a Webley patent. The book on WC Scott has a photo of the receiver. It's the most expensive boxlock they produced after the merger with Webley. It's a screw grip.

The simple way to tell if a gun is a screw grip is to look in the slot in the breech face into which the rib extension fits. You can see the threads.