The following chart details the Fender serial number schemes used from 1965 to 1976.
You will notice that there is quite a bit of overlap of both numbers and years.
As you can see by the overlaps of numbers and years, the reference to the actual production date is rather loose. The numbers and decals are produced far in advance, and apparently, some N9 decals, (which were supposed to be used in 1999), were affixed to some instruments in 1990.
As a result, you will see some 1990 guitars bearing N9 serial numbers. Fender has recently (in the last 20 years) introduced LOTS of different serial numbers schemes, depending on the country the Fender was made (USA, Mexico, Japan, Korea, etc). The following serial numbers are somewhat outside the more, well known Fender serial number schemes.
These books are the same resources we refer to here at Fender, when trying to research answers to these same history and dating questions.