I have been thinking that this slipcased version may be copies done by the publisher for the contributors. No way to prove that unless we track down Chuck Miller or one of the contributors. The pages are also arranged a bit differently. The signature page is on the left in the regular signed edition and on the right in the slipcased edition.
I think it is cool, Alan, that yours was also signed by George Romero. Do you know the story behind that?