Here is a picture of a porcelain insulator with a recessed top, maybe like Lee described in his picture. Mine is an M-3620A. The top is metal according to the guide, but mine is weathered so much that I'm not sure I would have known. And I don't want to clean it up or alter it in any way to verify it! I don't think these are very common, but who knows? The country is a big place, and maybe there are a lot of them someplace...