The Washer,plug,drain 12 is the lowest priced item on this Oil Filter - Oil Pump schematic.
Parts such as the Oil Filter - Oil Pump: Valve,relife, Case,oil Strainer, Pump Assy.,oil, Spring,element are included in the Cbx1000 Supersport 1980 (a) Australia Oil Filter - Oil Pump parts fiche.
In this schematic the "Bolt,flange 6x25" is used most often.