Hi Hans
On my Enfield some of the nuts are half the normal width to prevent fouling the stand and other bits around the rear wheel.
Just a thought - No doubt one of the M20 experts will let you have an answer.
Paul
I'm sure that Paul is right I bought a brake anchor bolt and nut from Russell's last year. The one that they sent me has a thin type head, approx 1/4 inch, with the thread length 1 inch. The bolt has a hole for a split pin and the nut is a castlellated. Hope this helped?
Dave