All seats from all models should be directly interchangeable.
ONLY difference between model years is color choices, and 2008's had a shitty rear sprocket with a rubber gasket on it, and some 2008's had a defective fuel pump.
Besides that there is nothing different between model years other than color.
Why do you want an extra seat?
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2008 WR250X
Gearing: 13t - 48t
Power Commander 5 / PC-V
Airbox Door Removed - Flapper glued - AIS removed
FmF Q4
Bridgestone Battlax BT-003rs