The Japanese-made Fenders do have some slight serial number differences (typically a "J" serial number prefix). I believe this was a mistake on Fender's part using the same prefix for both U. Below are some examples of letter prefixes used in recent serial number schemes.
Neck dating is useful in determining the age of a guitar, but is not definite.
instrument production history, production dates have been applied to various components.
Production dates have been penciled or stamped on the butt end of the heel of the neck of most guitars and basses.
So, obviously a neck date, while being helpful in providing a date range of production, cannot be a definitive reference.
While there have been periods where dramatic changes have occurred, for example: the transition periods between Leo's Fender and the CBS years, as well as the transition between CBS' Fender and the current ownership, generally speaking, most models are feature specific and do not change from year to year..