The Nut,flange,6mm is the cheapest item from this partsfiche.
This schematic depicts all components used in the Xr250 1980 (a) Usa Rear Fender partslist.
A sample from the products for the Rear Fender include: Fender,rr,*r-23*, Bolt,rear Pivot, Bolt,flang,6x16, Nut,flange,6mm.