As to the effectiveness and/or usefulness of a keyed washer in bicycle headset specific applications, I cannot give any opinions. It is helpful when discussing useless features with others to use similar terminology, and that's why I introduced the term, albeit late in the discussion. Keyed washers may just be a remnant of some other application in machining or some such thing (I would certainly expect keyed fittings in some timing cam applications, where orientation is essential), and many reflector brackets that interface with the headset have these keys to keep them oriented properly relative to the steerer. This type of reflector bracket functions essentially as a keyed spacer/washer. That's probably the historical basis for some threaded steerers having keyways. As long as there's a keyway in the steerer in order to accept a possible reflector bracket, one might as well manufacture a keyed washer for this application. They certainly aren't as ugly as the type of reflector brackets to which I referred.