Those wheels are "wagon" type wheels and require no hub cap. Maybe a hub center, not a hub cap or wheel cover. The hub center would cover the very end of the axle. And the hub centers that I have seen in the past are chrome over plastic. Earlier versions were chrome over metal.
Or the wheels in question can be used with no other pieces. I have the same wheels on a boat trailer and at the end of the axles are fittings that allow me to grease the hub with a grease gun.
1966 Mercury Park Lane 4 DR Breezeway 410 4V, C-6, 2.80 - Streamless.
1966 Lincoln 4 door Convertible 462 4V 1971 Ford LTD Convertible 429 4V Phoenix ~ Yeah it's hot however it's a dry heat!