The hub caps ( stainless steel simulators ) were manufactured by Dicor ; and are no longer produced .
Retention method varied by rim size and chassis , there are at least three

I believe those on the E Ford chassis are a push fit ; retained by " claws " around the outer edge.
Check all the fake lug nuts for a stamped oval or slash ( there will be two on opposite sides , if there are retainers .
The oval mark indicates they are a cover for the actual retainer , the slash means they are the retaining nut and have to be removed with a socket .
Special plyers are available for the retainer covers with an oval .