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.
Most notably, production dates have been penciled or stamped on the butt end of the heel of the neck of most guitars and basses.
As you can see by the overlaps of numbers and years, the reference to the actual production date is rather loose. The serial numbers do not reflect the change, as Fender continued to make instruments using existing tooling, parts, and serial number schemes.
With 1990 came the introduction of the "N" prefix serial numbers, which stood for the 1990s.
MADE FENDER STRINGED INSTRUMENT For the majority of Fender's U. instrument production history, production dates have been applied to various components.
While this neck dating is useful in roughly determining the age of a guitar, it is certainly not definitive.
In January of 1965, Fender was sold to the CBS Corporation.
The serial numbers do not immediately reflect the change, as CBS continued to make instruments using existing, tooling, parts, and serial number schemes.
You will notice that there is quite a bit of overlap of both numbers and years.The following chart details the Fender serial number schemes used from 1950 to 1964.