You're right Karl, they had a hard time with that right from the get go. It's just to prevent oil from leaking out. If you look around at various pictures, you'll see tape on anything from -1s to -1Ds.
It's quite well done, certainly better than mine of the same scheme! Don't think the paint usually faded that much on the later war aircraft but still cool. Also I think that Vb Spitfire is from a Canadian unit, can't recall which one though.