So I took a caliper to the shaft and found the major thread diameter to be .562" or 14.2mm
I looked on the metric thread tables and found M14 too small and M15 too large.
I then looked on the SAE tables and found the following:
.562" corresponds to 9/16" - 12, 18, and 24 tpi ( or 9/16" BSP - 12 coarse, 16 fine)
Does this make sense to anyone? This is sort of killing me. Nothing on the web corroborates the use of a 9/16" thread on the steering column, yet this is what I have on my hands.
I must have misplaced the original nut, because the one I *thought* was correct is most definitely not.
A 9/16" nut has roughly 23.9mm across the flats, corresponding to 24mm, which corresponds to one of the 3 you gave