The lowest price component in this Turn Signals (h1/a Optional) parts-list fiche is the Nut,10mm.
A sample from the products for the Turn Signals (h1/a Optional) include: Washer-spring,10mm, Nut,10mm, Washer-plain,6mm, Lens-signal Lamp.
In this schematic the "Screw-ovl Cntsnk,3x15" is used most often.