The "FS1-E" name was derived as part of a series of bike models and model numbers YAMAHA produced starting with the "AS1-E" which was a 125cc twin from the late 60's. The "E" part signifying a bike destined for the English market with English specification on things like lighting and size of rear number plate holder etc. There were also the AS1, AS1B and AS1C models produced around the same time for other countries. There was not a BS1 or BS1-E as far as I am aware but there was both a CS1-C and a CS1-F as well as a YCS1-E. Other models following the model prefix pattern include CS2-E, CS3-E and CS5-E which were 180cc twin and 200cc twins respectively which eventually turned into the RD200.
The first FS1 was produced around 1968-69 and had a model prefix of "260" as well as the later "424", also frequently seen as as a part number prefix in many FS1-E parts book. The FS1-P (model prefix 283 & 378) was the next fizzy to be produced which is the model YAMAHA UK keep telling me is what we got over here in 1973, but I keep trying to tell them it was actually the Dutch specification model and we had the "SS" then shortly thereafter the infamous "FS1-E", both with model prefix "394" from late 1972 onwards. These first "SS" badged FS1-E's were built using a YB50 frame according to the Part number in the Parts List from October 1972. YAMAHA also list model prefix code 470 as a FS1-E but I have never come across one of these yet although I have a Parts List for it
Others FS1 models include:-FS1-C (Canada)(557),
FS1-F (France) (469 & 599)
FS1-H (Belgium) (325, 350, 380, 1J6), FS1-HA (472),
FS1-K (Denmark) (381), FS1-KS (471)
FS1-G (Germany) (468),
FS1-N (USA & Canada) (473).
FS1-P (Holland) (378 & 379)
FS1-S (Sweden) (381)
There was also a 1975 mainland European (Austria. Belgium,Switzerland, Germany, Holland & France) model FS50 (474 & 475) with "SS" side panel decals made similar to the FS1 but with a 5-speed gearbox, autolube, odd shaped tank and footpegs. Came in Baja Brown, Competition Yellow and Balboa Blue.
Other bikes with a similar sequence of model prefix codes include CT1, FT1, HS1, JT1
Hope this helps clear things up for those of you still awake!!??
I will add photos to this post at some point when I have less time
