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.
The neck date simply refers to the date that the individual component was produced.
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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.
Most notably, production dates have been penciled or stamped on the butt end of the heel of the neck of most guitars and basses.
instrument production history, production dates have been applied to various components.
(If you are not comfortable performing this operation, please use an experienced professional guitar tech in your area).