My three mid-70s Fuji I have bought new had the Showa year type serial numbering system. All were built in 1974 but the last one was most likely a 1975 model year since it was built so late in the year (September) and a low sequential number
Special Tourer F9B18529 bought May 1974 - stolen at four months old, replaced with:
Special Tourer F9B18546 - sold in Oct '76 so I could upgrade to scratched floor model
Special Road Racer (S-10S) K9I003xx (which I still have so keeping last digits hidden)
I'm guessing that the first character is the manufacturing plant.
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'75 Fuji S-10S bought new, 52k+ miles and still going!
'84 Univega Gran Tourismo
'84 Univega Viva Sport
'86 Miyata 710
'90 Schwinn Woodlands
Unknown brand MTB of questionable lineage aka 'Mutt Trail Bike'
Plus or minus a few others from time-to-time