On this parts fiche the Nut,flange,6mm is the most specified component.
The most reasonably priced component used on the Sfx50 1998 (w) Portugal Mp Seat/luggage Box is the Bolt-washer,6x12.
Some items from this specific schematic: Seat Comp,*type3*, Rubber Oil Seal, Stay Rr Inner, Box Assy,lug*nh1*.