Those look like OZ Racing wheels to me. I had a 15" set which were body colored in Anthracite Grey. I remember buying center caps for the wheels from OZ Racing and I would assume they sell the key as well.
Check with them, they are actually a very large manufacturer of wheels for other aftermarket wheel companies like Fittipaldi, MSW etc.... Their customer service was excellent and pricing was reasonable as well. Do a search on google and put in O.Z. Wheels.
The key is basically a solid chrome cylinder with a smaller cylinder on the end of it offset to one side. Mine had a red handle similar to a screw driver.
Hope this helps.
02 C32 AMG
84 190E 2.3 5 Spd