Nice work John. Now that you brought it up, I have a 66 Impala 2 door. I believe the car was delivered new with the small caps because the wheels are painted body color - white in this case. When I bought the car 38 years ago, it had the standard Impala full covers. So they were probably (maybe, who knows) added at the dealer when new or early on.
Since the Camaro/Nova wheel covers were mentioned above, I think the 63 SS spinner caps were offered on the 68(?) Camaro SS.