If you have the box check the "Spec. Ord." number. This is usually the Julian or "Ordinal" date the gun was originally "put up" or assembled. The first digit is the last of the year the gun was made, and the last 3 the day of the year. You do need to have an idea what decade the gun is from though.
Example, 6160 would be the 160th day (June 9) of 1986 in your case.