They have been placed at the top of the neck plate, on the front of the headstock, on the back of the headstock, and on the back of the neck near where the neck bolts onto the body.They were stamped on the back vibrato cover plate (early '50s Strats), and on the bridge plate between the pickup and the saddles on some Telecasters.(If you are not comfortable performing this operation, please use an experienced professional guitar tech in your area).
There were periods of time when this was not consistently done, (between 19), and there are certainly other examples of short periods of time, and individual pieces, where the dating was simply omitted.
While this neck dating is useful in roughly determining the age of a guitar, it is certainly not definitive.
You will notice that there is quite a bit of overlap of numbers and years.
The only way to try to narrow the date range of your specific instrument would be to remove the neck and check the butt end of the heel of the neck for a production date, which may be stamped or written there.
If you have what you consider to be a somewhat "odd" serial number, please check the following chart to see if you find your serial number configuration here.
This chart contains If you are unable to place the approximate year of manufacture of your instrument using the above charts, there are a few great books available, which have invaluable information on the history of Fender instruments.
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.
The following charts detail the most common Fender serial number schemes used from 1976 to the present.
Please note the introduction of the "S" prefix serial numbers.
But once again, due to the modular nature of Fender's production methods, and the fact that most serial numbers schemes are not sequential and usually overlap from between 2 to 4 years, (from the early days of Fender, through to the mid 1980s), dating by the serial number is not an exact science.