I bought the seats from an aircraft "breaker" so it's always possible the seats are not from a 150L. I can only tell you what he told me: seats come from a Riems 150L. I asked for the serial number and he gave me F1501084 but said it may have been F1591084; F1591084 seems very unlikely.
If F1501084 is correct then it was manufactured in 1974, and according to the parts diagrams the seat is a different design to that in OZ. Indeed, there seem to have been at least 3 seat designs for Aerobats, and even more for standard 150s. As you say, seats for French manufactured aircraft are the same as those for US built Cessnas.
I would have no problem with a different design so long as it fitted; it's the distance between seat runners that came as an unwelcome surprise.
I will check the seat runner measurements against a FRA150L built in 1974 which has a different seat design to that on OZ.
