Well both, really. The caps I found on ebay were Mercedes AMG caps with 57mm ID and 60mm OD. When I went to install them, they fit in the center hole with slight resistance, but nowhere near enough (they actually fell out on their own).

I noticed they use the same style tensioning "C" spring as the OEM caps, but smaller. I carefully swapped the spring clip, which forced the locking tabs out a little more, which then held tight. One issue, the caps on the rear rims will not come out, but they rattle slightly. I got a large diameter O-Ring and used that as a "shim". I need to find a slightly thinner o-ring, which will completely hide it from view. The front actually fit in tight with no play at all. Weird.