Salopian knows who has the records so he may spot the thread. The date code is usually in the form of one or two very small letters contained within the small crossed sceptres mark. I can't see the mark in you photo. Your proof marks are 1925 to 1954. They started date coding from 1921 then stopped for some reason from 1941 to 1950 then started up again. If you cannot find the mark and letters it may be from this interim period.
Birmingham Gunmakers by Douglas Tate may be the book that you are looking for.
Nice condition gun. Lagopus.....
