On this parts fiche the Washer,32x6.5x2.3 is the most specified component.
The most reasonably priced component used on the Cn250 Helix 1988 (j) Switzerland Kph Seat is the Washer,screw 6x12.
Some items from this specific schematic: Bolt,flange 6x12, Collar,l.grab Rai, Collar,r.grab Rai, Rail Comp.,r.grab.