509-11631-01-96 is a Yamaha vintage standard piston that fits the following: 70mm
DT2MX - 1972
MX250B - 1975
MX250C - 1976
MX250D - 1977
MX250E - 1978
YZ250C - 1976
YZ250D - 1977
YZ250E - 1978
These are vintage models and so are the pistons
please not you may have to pay import duty if not in UK