Bought a cam cover gasket set for my E39 528 - single VANOS - and as well as the main gasket and seals that go round the plug recesses, there was a seal like you get on the front of the crank etc but about 1 1/2 inch outside diameter. With nowhere obvious it would go. Although it looked about camshaft size. Any clues? Just for interest - my engine is now oil tight again.