Glenfiddich markings and age
 

Can someone tell me how to determine the age of a Glenfiddich bottle by the markings on the bottom? I have a bottle with the numbers 4476 on the bottom and I know it is old because my Godfather used to bring back liquor duty free when he used to fly to the Caribbean Islands in the 60's, 70's, and 80's. Thank you for your insight.

The markings on the bottom of all bottles are the mould marking made when the bottle is manufactured. The only way to possibly date a bottle is to post photographs