Originally Posted by SBinNYC

That washer used to be keyed and there used to be a lengthwise slot in the axle threads. The keyed washer fit into that slot. The object was to prevent the cone from turning when the locknut was tightened. You could place the cone wrench on the cone on the opposite side and tighten locknut with a wrench. This made it possible to tighten the locknut without either the cone or axle moving and use only two hands.