These are the special 'Reduced Hex' nuts, as originally fitted to the rubber pad (square) adjusting screw A11/817 - CNC manufactured in stainless steel to the original Norton pattern with Whitworth spanner size.
Price is Each
Like so many other Norton engine fastenings - this nut was specially made for the Norton SOHC cambox adjusters - the hex being a smaller Imperial diameter (across flats) than would have been commercially available for that thread size. In so doing, it allowed easier access to adjusting the square headed rubber adjusting screws, as a smaller spanner could be used.
We CNC manufacture these nuts to the original Norton dimensions - but in Stainless Steel, and we even mill the Hex (as it is now a difficult Imperial size to obtain commercially) - so although small and mostly hidden in use, they are very pretty to look at! (and this explains why more expensive than standard stainless nuts)