So it looks like the nut is a "standard" thickness (5/16") nut but with no bevel.
I ran across this Grade 8 yellow zinc plated nut (yes, I know major overkill!) at Pep Boys last night. I realize that is not cad plated and in light of your recent post, has the bevel on it but it gets me a little bit closer in appearance to having the correct nut.
I suppose I could go to my local Chevy dealer and order the superseded nut, strip the plating and then have it cad plated. I was just hoping for an easier solution.
Carlisle is only a week away so I'll have a look at some cars there to see what is being used.
Maybe other members could also chime in and post what is on their car.
Many thanks for all your help!