That looks like the half moon seal. There are two of them, one at the rear of each cam and they sit in the head. They are not part of the cam cover. The good news is the thing is too large to go down very far. Mechanical fingers should get it out in no time. No mechanical fingers you say? Take an electrical test probe or anything with a point on it, even a coat hanger that's been sharpened, stab the sucker and pull it out. Since they're made of rubber, they shrink a bit over time and should be replaced anyway from time to time.
Series 2 USA 1750 GTV (in Series 1 European clothing)