Genuine Aprilia water pump oil seal for '00-'02 Mille, Mille-R, '04-'05 RSVR and Factory, and '06 Tuono.
This is the oil seal that is situated on the water pump impeller shaft. It prevents coolant from mixing with oil.
Aprilia part no. 0650315 (ex 0650310)
Part no. 12 in the diagram