This is a genuine Rega tonearm securing nut which is used to secure the older 'single point' bolted tonearms to a plinth or armboard. If your Rega tonearm has a large threaded base (rather than the latest 3 point screw fixings) then this is correct nut to secure it in place - please see the last two pictures to see the nut in use.
There are too many models to list, but this nut is suitable for the RB200, RB250, RB300, RB600, as well as some Rega based arms from other manufacturers, i.e. Michell, Goldring, NAD, Moth, Origin Live, Audio Note and so on.
