Well, this universal “truth” was debunked when we found a bunch of amps with transformers made by the Better Coil and Transformers company.The decade letter codes break down like this: S = 1970s, E = 1980s, N = 1990s, Z = 2000s.
and these changes are often disclosed and of a non-malicious nature.
In the silverface era Eminence, Oxford, Utah and Rola were more often used and Jensen was phased out.
Even at we struggle with keeping a correct amp and speaker model overview.
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.
Japanese Serial Numbers on Peghead Decal Note the lack of S, E, N series.