Thanks Thor. They say "never say 'never' or 'always' " with Lee Enfields, but having said that I think it unlikely that a Mk2 scope dated the year before would have been originally fitted to a 1945 dated Shirley rifle. What you say about the butt being swapped over & it originally having a Mk2 scope is possible, but I think unlikely. If you want to see the serial number of the rifle your butt was originally fitted to, then if you take it off the serial should be there on the part that is normally hidden inside the butt socket of the rifle. It may well be original to your rifle - & would fit in with the Mk3 scope serial number. However, to remove the butt you will need an armourer's brace, or (as Peter might say) a fXXXing big screwdriver!

ATB.