Very Cool Book I think 2 more of those are known to exist right now. As for Keys I have seen a CGC 5.5 copy of Amazing Fantasy #15 with a Double Cover sell on ebay many years ago and a cool note the guy who sold it back then posted about it here just this year. I am a big ASM Double Cover guy so I have seen a lot of key issues and I even have a few like first Green Goblin, Kraven the Hunter, the Rhino, the Prowler, Morbius, The Punisher & Carnage come to mind. I would guess almost all older comics before 1980's had Double Cover copies printed the question is how many still exist.