Hi Danny
Welcome to the forum, Have a look >>> HERE <<< at the letters beginning with N (For .177") and T (for .22"), You should get a date to within a couple of years!
I was out your way this morn at Tansley bootsale and saw a Meteor, It looked old, He wanted £75 for it, It didn't look too bad but I already have a mk1 and mk2
Cheers, John