Thought so.

When you see it, examine it carefully for other markings, especially on the underside of the barrels, on or near the fore-end loop. Look for a five-digit number stamped. Also, take note of the serial number of the gun, presumably on the guard tang. The number on the tang will probably also be stamped on the underside of both barrels. However, there should be a third number, probably non-matching, on or near the loop. It's possible everything will match, as Lyon & Lyon was strictly a retailer, and may not have assigned their own serial numbers, in which case they WILL match.

Also proofmarks, should be Birmingham. Should be crown over BV, crown over BP, crown over NP, "Cordite xx - xxxMAX"; xxxEX, should all be present on the barrel flats. MAY also be 3.xx". I don't have to tell you that bore condition is paramount as to value. Photos will help. Please let us know what you find.