I got some 13" x 6" wheels from a friend off a BMW 2002. They did not have any center caps. The only center caps I could find to fit were Enke centers. I changed out the Enke centers with some Opel ones from Ebay. The three pictures show the Enke cap, no cap, and the Opel cap.
I will post a picture of the center on the wheel soon.
Interior Exterior Engine 73 GT Opel Home Texas Opel Carlisle '99