I found a Ford Racing center cap at Official Site of Ford Racing Performance Parts - Mustang Parts, Crate Engines, Packs, Wheels, Superchargers, Racing Parts | FRPP - Performance Authority. Looks like it's meant for the 17" 15 Spoke Fiesta Racing wheel. Has anyone purchased these caps and / or thought about using them on 15" steelies? I have no idea if they'd fit or not.
Link: FIESTA FORD RACING CENTER CAP | Part Details for M-1096-FA* | Ford Racing Performance Parts
Link: FIESTA FORD RACING CENTER CAP | Part Details for M-1096-FA* | Ford Racing Performance Parts