Determining the date can also be important from a collector’s perspective, since the pre-1966 vintage Fender guitars are generally considered the most valuable.
Fender was sold to CBS in the first week of January 1965, but already in late 1964 mass production was slowly being introduced, which gradually lowered quality and ultimately collectibility.
Regarding quailty, I have owned many of both these Japanese guitars and it is a fallacy that the than the Mexican made guitars and rival many of the USA models.
The JV and SQ guitars, as well as some of the E series, had USA parts (mostly pickups, switches, and potentiometers) that were shipped over to Japan to help speed up production while the new USA plant was being set up in Corona, California.
The serial numbers do not reflect this change - Fender continued to make instruments using existing serial number schemes.
The new Fender did not acquire any physical assets of the old company, just the name "Fender".
The same goes for bass parts and basses from the first half of 1965 are often considered as good as early 60s basses as the real changes occurred later than that. There are tons of signs to look after when trying to date a vintage bass, but of course the serial number is a good place to start.