I'm going to guess they are from the Fox Body Mustang "Ten Hole" (aka "Phone Dial") wheels.
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Arent these caps flat and not bow out like the caps in question?

truck's lookin good buddy, can i nominate you yet?
I think they bow out a little. I have a two of these rims I kept for some drag radials, so I'll dig out the center caps and look.
my caps are also plastic, but I know they all came with numerous different sets of caps.
I've seen those before. I am pretty sure those only came on the 4 cylinder towards the end of the Fox Body run. Mine are metal.
So were they on the 14" wheels from the 4cyl?