Special Tank washer AP8150536
Fitment - RSV 04-10, RSV4 09-17, Tuono V4 11-17
Genuine Aprilia motorcycle tank washer or 'Rosette', part number AP8150536, item number 9 in the 1st diagram.
This is used to protect the tank when tightening the front mounting bolt as well as act as a normal washer.
It is also a washer for the fuel vapour recover system on the Tuono V4, item 23 in the 2nd diagram.