Although I've not looked in the trunk of every Cuda out there, but this is the 1st I've seen or heard of this kind of palnut used on an E-body.
I've seen something similar used on various B-bodies though( different sizes)
70-71 Challengers are 10-24 machine thread

But these ? maybe production date or plant thing ?
8-32 machine thread - same nut as used on the sidemarker lamps.