Ivor,
I think one dfference between the nut in your picture and the picture from Bill, as well as the picture from deGroot (below), is that the points on your nut are not chamfered where they come against the rod cap as are the other two. The unchamfered points seem to contact the recess in the cap. I will, and should be, corrected if I'm wrong, but I think washers are not used under the type nuts we're showing here (Triumph type?).
Richard L.