Ummm.... you're thinking too hard....
You have the baby moon caps for something like the Stockton wheel aftermarket wheels, like I have on my Model A. These caps will only fit aftermarket wheels.
http://waywardgarage.com/me/1931FordCoupe/2007/03.03.07_01.jpg My '57 has stock Chevy steel wheels from a '69 or so truck, but they look like your wheels. I used dog dish caps that fit the three 'nubs' in the center of the wheel. Take a measurement, I can't remember what mine is, and check out eBay, or Mooneyes, they will have a simple baby moon that pops on your existing wheel.
http://waywardgarage.com/me/1957ChevyTruck/05.22.07_57_01.jpg
