It's not a 500F or a 550F seat. The hinge plates are wrong, there are rubber inserts missing which the 500 and 550 use. May be a 500 twin though but can't recall how that seat looked from the base, do seem to recall they were brown though but that may just have been on certain colour bikes.
Don't know why but I think it's a CB250 seat, maybe K3 or K4 seat