I have one on my Rosso Corsa Lemans...Having owned a lot of Aprilias I recognized the cap .
Order the tank ring for a 2002-2005 Aprilia Tuono...same exact thing..went right on in 10 minutes..Got it from AF! racing
https://www.af1racing.com/store/Scripts/prodView.asp?idproduct=94017&sku=TW-TRT.00.640.17500.B&description=SW+Motech+Quick+Lock+Gas+Cap+Tank+Ring
I have the Quick lock " Evo Daypack" and the Quick lock "Evo SPORT" tank bags got them from Revzilla
Love them
KINDOY2's post in Anyone successfully use one of those quick-release tank bag adapter rings? was marked as the answer