There are 2 revisions of the mid 2012 retina display unit, you can confirm this by apples unique part numbers (661-6529 is the original unit) This unit has been replaced by a second revision that is stated as being compatible with the Mid 2012 and the Early 2013 model (part number 661-7171).
You dont have to worry about getting the earlier revision though, because it has been discontinued and replaced by the second version... just check when you are purchasing it from whatever company you decide on...
'''Please note:''' apples part numbers are never written on the items themselves. they are purely for reference purposes, found both on the outer packaging (when ordering direct from an apple service centre) or on their apple database, known as GSX (global service exchange)
i hope this helps