This is the top gearbox nut that is fitted onto the Upright Adjuster Bolt (Item 0566), that is loosened and tightened to adjust primary chain tension on all upright gearbox models. These are CNC manufactured in stainless steel. Price is each.
This version is copied from an original pre-Featherbed Manx Norton type nut - being slightly lighter than the original roadgoing type (which was just solid Hex with a thread through it), but there is no reason why this version cannot be used on all roadgoing models as well - it works just as well and looks prettier! can be used on all models. Manufactured in stainless steel.
As a final point - the hex on this is a lovely correct fit for the Whit spanner, so if your existing nut is chewed (as so often they are), this will allow much better purchase and less slop in the restricted area that this is fitted - normally a real pain to adjust!