Unstable Spoilers are apparently starting in full force. Apparently, a number of people have received pieces of oversized cards in the mail from Wizards of the Coast. Specifically, these:
That's right: Contraptions are being revealed at last!
We do have all the pieces now, so this is what we know:
The lack of mana cost would seem to imply that Contraptions do not exist in the library; they are likely, as was highly speculated, a separate deck in the command zone, and are assembled randomly (or perhaps not randomly, but still). You'll need to place them on a "sprocket" to determine how frequently you are able to "crank" them. Very unique!
Also note that they are borderless. MaRo has hinted there is something special about their art (probably it all fits together like one big contraption) but perhaps the lack of silver or black borders could mean special rules to allow it in certain formats or under certain conditions, without messing with the legality rules of silver-bordered cards. Maybe. We can dream.
Worst case scenario, casual EDH groups will be able to use them, and that's hardly a bad thing.