The "floating J" serial numbers are confusing. A typo in the SCSW 3rd makes it even more confusing. Here's my best explanation of what I believe is correct.
1971-1972 from serial numbers 1J1 to 999J99
(note there are up to 5 numerical digits plus the floating letter J)
(also note I've added a leading 1 to J1 that was omitted in SCSW-3rd)
1973-1981 from serial numbers J100000 to J915400
(note the factory went back putting the J first but expanded s/n to J plus 6 numerical digits)
In 1983 - resumed the "floating J" starting with 1J10000 but this time with six numerical digits plus the letter J.
In 1984 - change to 3-alpha/4-numeric format . . . so I guess the floating J sank
As others have said, only a factory letter will determine the exact ship date.
Hope this helps,
Russ
edit to add: the serial numbers reported in the pre-J prefix era (prior to 1969) are just examples of serial numbers with known ship dates. There was no attempt made to provide an inclusive range because such data was not known or compiled at the time of publication. Many collectors have kept track of the known ship dates since the SCSW was published and can interpolate ship dates a little closer than the examples in the book.