It sounds like a pre-War Breakdown Pattern, made from 1932 to 1939 Ron. The model details would have been acid etched into the top of the cylinder originally but have since worn away. If the smooth rust finish you describe is just age 'patina' then it's probably wiser to leave it as is. Unless your friend is determined to bring it back close to its original blued factory finish. Most collectors take the view that it's only worth restoring older airguns if they are 'near death'; that is, heavily rust pitted and perhaps in need of mechanical attention too.