Thanks for the extra effort Jeff. The ' Library 'section, supplied by Jim, lists this part as 33771 S4. Zinc-Phosphate Brown/Dark Grey. I'll be using the ' rounded/angled ' nut. The one on the right in the photo supplied by the very same Jim. Brian
I'm going to go out on a limb here and question the 33771 part number. The May '75 MPC illustration does show that part number for the nut, but I don't believe it represents what was used on the assembly line. If you look closely at your photos and Jeff's photos of both the "tall" and "short" original nuts, you'll notice they have crimp marks. These are known as "distorted-thread lock nuts" or "crimp lock nuts". The crimps are added during manufacturing. The crimping causes them to grip the bolt more tightly, which is highly desirable in a highly critical application like attaching suspension components.
Part number 33771, however, is a UBS (Uniform Bearing Strength) nut, which to the best of my knowledge was never used as a crimp nut. You will note some visual differences, too - it is tall like the "tall" nuts, but has the top edge chamfer like the "short" ones.
Update: UBS nuts are "prevailing torque" fasteners, i.e. they also have distorted threads. But they do not have an obvious crimp like the other two being discussed. The large chamfer at the top is a result of the distortion. Note that the "short" nut being discussed has a large top chamfer, but it also has visible "tri-bar" (my term) crimp marks. The "tall" nut has a rectangular crimp on one or more of the flats.