UEFA Champions League Match Attax 2016/17
Following on from Daniel Roe sending scans of the two different versions of the Lionel Messi Gold Limited Edition card I did some research and found some 6" x 4" uncut sheets with blank backs on eBay.com. I've no idea how these were released.
UPDATE (18-02-2017 17:11): Daniel has confirmed that this sheet is inside the Two-Player Starter Box as a "dummy" card - it's the card you can see through the box window. The proper Limited Edition card is inside a wrapper within the box.