Buy the center caps new from BBS directly, last time I did it was ~$400/set total, and worth it IMO. Make sure to get the correct color. There is regular silver, and a Motorsports version that is a slight champagne color, almost smoke silver. I have a set of each. I gave up trying to find used caps, they are always in poor shape and overpriced. And the VW versions don't fit (different size threads).
RENNtech may have installed them, but you'd need to ask Hartmut to verify. Any other visible RT items on the car? I'm guessing no 6L.