It's just something I had laying around that I used to cover that ugly hole in the Mercedes engine cover. You would be amazed at the number of inquiries I got on what it was and where to find one. In fact, I found a second one and gave it away to a member of this board.
For anyone looking for one, try your luck on ebay mid to late 80s Peugeot 505 alloy rim wheel center cap. When you get it, break off the plastic retaining tabs, put "GOOP" or similar adhesive around the edge and stick on your engine cover.