A small lesson learned today. If you fit a tacho to a plate held on top of the fork by the large fork nut, do not do as I did and put it on the left.
Norton hung the optional tacho on the right and now I think there is another reason apart from improving the cable run. When on the left, after a few dozen miles it gently vibrated round backwards, anti-clockwise, and took the top nut half a turn backwards with it. It would not happen on the left!