My Dad had these among other 69Z parts, but I can't find any mention of them in the AIM or elsewhere.
Originally as I was sorting through everything after he passed away, I'd thought they were part of the upper control arm water/dust shields. Clearly I hadn't looked at the shields!
So now I'm completely puzzled what they are and why he might have saved them.
It is entirely possible they're from another GM car since he worked for GM his entire career.
Dimensions might not be entirely clear, but VERY roughly they are: