Very lovely gun. Rebounding sears, built by Webley Scott. Should have both Birmingham and London proofs. A quality boxlock, you can see from the fashion of carving on the fore end and the rebounding sears. One of a pair. I think William Evans quit using William Evans from Purdey in the mid 20s. So your gun was made around mid 1920. There are others here more knowledgeable then myself here. You'll soon have a more exact description as it is a fairly well known gun.