It seems to be something intended mainly for service personnel for use in combat areas, and will be restricted therefore very collectable. If anyone knows a GI Joe then that’s probably the best way to get a deck or two!
They're far more common that you would think and I've seen them on ebay at some reasonable prices but you have to keep looking and bidding as they sell rather quickly.
Member of the Magic Circle & The 2009 British Isles Close-Up Magician of the Year
It's not really an optical illusion - it just looks like one!