Just oil that trickled down from the head as I was fumbling with the head removal.
Here's an interesting thing I noticed when I inspected my head one day a while back. Notice that a 12" long section of the top of the head, about 1.5" from the valve cover mating surface, has been milled and the milling cut through the cast-in letters:
I just sent the pics to Charlie and he checked 2 other unrebuilt engines and they had the same milling marks. He surmises that this was done for some reason at the factory.