What measurement do you have across the flats of the nut?
Presuming it is a 1969 or 1970 model then the thread is 3/4 x 20 UNF. There was a special spanner part 61 6062.
In the back of my mind something tells me it takes an 11/16 whitworth spanner which is 1 3/16 inch (about 1.2 inches) across flats, rather than an AF spanner.