This locking nut is the Stover type, often known as a Philidas nut. It is 11/16" AF. Height 0.4375 (original spec=0.425" +/- 0.015"). Use this type of all steel nut instead of Nyloc Nuts where a stronger grip is required, or the area of use is subjected to heat which would melt the nylon in a Nyloc nut.

This part is listed on the following page of the website, where further information and background reading may be found.

This part appears on the following Triumph Catalogue Plates