The stock is for the Webley No.1 Mk1 flare gun that uses the same frame as the Mark VI .455 pistol. The frame has a groove machined into the lower part of the grip area. A matching tenon on the stock plate aligns the stock to the frame during recoil. Many people have ground the tenon off or poorly cut a groove to make something to go with the pritchard pistol bayonet. I have seen no published data to legitimize this stock configuration.