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 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.
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.
Most notably, production dates have been penciled or stamped on the butt end of the heel of the neck of most guitars and basses.