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.
The biggest tip off would be the control panels which brings us to interesting factoid #2.
After CBS bought Fender in January 1965, there were still plenty of control panels for various models that were in stock.
The factory realized its mistake in February because these amps have a "PB" date code in black ink.
Now, we obviously haven't looked at every amp made in January 1966 so this isn't set in stone.
These say "Fender Electric Instruments." Depending on the model the use of these pre-CBS panels have been observed on amps as late as August 1965, except for Champs and Vibro Champs which had foil stickers on the back the chassis.
Fender must have had a million of these labels printed up since they appear on Champs and Vibro Champs well into 1966.
New panels made after the CBS acquisition were used beginning in April 1965 and say "Fender Musical Instruments." So, if you have January 1965 amp, it will have a Fender Electric Instruments panel whereas a January 1966 amp (even though it has that green "OA" date stamp) will have Fender Musical Instruments. Although the database doesn't have thousands upon thousands of entries, we are seeing some interesting patterns emerging that will help date amps by serial number. The early amps (woodie and tweed) had serial numbers handwritten on the tube chart.
These appear to be used sequentially independent of model.
In March 1985, CBS sold Fender to a group of private investors.
The serial numbers do not reflect this change - Fender continued to make instruments using existing serial number schemes.
Dating a guitar or amp can be a very exasperating experience.