16 of those "heads" in the Mex. Bomb. / NY City stile are shown in the Necro 2 Book, or at this website: http://michalovski.free.fr/page35.html
the 2 I own, are there too. I once asked Giger about, and he told me there are maybe 20 f them, but for sure not much more. Most went to friends of him, nearly all were dedicated to somebody. Originally they were painted directly on the first page of the ny city books, but beeing original paintings most people took them out later on to have them framed and displayed.
The one for sale I got some years ago, the other one I got only recently. Only because I now own 2 of them, I agree to sell one; I know several of the other owners of those paintings, but as they were dedications, everybody has only one, and nobody wants to sell them. Thimoty Learly too used to own such a painting; his isn't shown in the Necro book.
As I'm pretty used to one of them, it's the one I want to sell; both are good paintings, but I admit, the one I want to keep is better framed, and has a painted background too; this makes it look eventually better than the one for sale.
But as soon as you don't compare the 2 of them, each is a good painting on its' own.
I must admit, I could not have such a large collection, if I would not up and then sell something; it would cost me too much; so I agree in selling sometimes out of my collection, to get space and cash for new aquisitions.
but as I own one of them already for a long time, I prefer to sell first this one.