Forgeries
Dangerous forgeries have been made by chemically erasing the upright head or the frame and then printing over it. These can be detected using "black light" and other techniques. One of these fakes appeared in the Masson sale, and one or two in the Ferrary auctions. Some clever forgeries purport to show an inverted head with different head dies, which are obvious, and crude forgeries are plentiful.
Read more about this topic: Inverted Head 4 Annas