How do you determine the age of a Remington 514 with no serial number?

On the left side of the barrel there should be two stamped letters and a number. The first letter is the month code and the second letter is the year code. The Remington Society website contains a page that contains a listing of the date codes and their corresponding months and years. For example, I have a model 514 that has no serial number and its date code letters are X M. X means it was manufactured during the month of December. Since there is no serial number on my rifle, I may safely assume that it was produced prior to 1968. The M can either be 1921 or 1965. Since the model 514 was not introduced until the late 1940s, I know my rifle was manufactured during December, 1965.