J3,
You didn't ask but here's what I can tell you.
FG-331
1977-1981
000 body
truss rod adjusted from the headstock
inlay down the center of the backSJ-180
1981-1985
000 body
truss rod adjusted from within the sound hole
no inlay down the backOther then that I think they are the same
Before the FG-331 was the FG-150 and before that was the FG-110. The later had back/sides made from agathis not mahogany.
Sorry for correcting you but the FG 110 and the Fg150 were around at the same time. The 150 was an OM sized FG180 and the 110 a smaller version of the FG140. The numbering on old Yamaha's reflected pricing. An FG150 would be a MSRP of $150. Country Joe McDonald played and FG150 at Woodstock BTW. I have an SJ180 and it's dimensions are bigger than an 000. It would appear to be Yamaha's own design.