Piece of cake to tell if it's a screw grip: Look into the slot on the breech face where the rib extension fits. You should be able to see threads where the rib extension engages.
My best guess is that your gun is an Anson & Webley, made by P. Webley before the merger with Scott. I have one of those made for William Evans in 1901. (Webley and Scott continued to make those Webley screw grips even after the merger. Until 1940, in fact.) My guess is based on information and photos in "The History of W. & C. Scott Gunmakers" by Crawford and Whatley". I'd recommend purchasing a copy of that book.
Mine is slightly fancier than yours with deep carving on the fences, but otherwise they're very similar. Mine is the Model 51, which sold in 1906 for 58 pounds. Yours is quite likely the Model 52, selling for 48 pounds in 1906. Features, price, etc seem a good fit. The higher grade Webley screw grips, like mine and yours, used a doll's head rib extension. On the less expensive screw grips, the rib extension was straight.