They are different. While the key caps or the same (the part that has the keys markings and snaps off). The key mechanism is different between the series and age of system. Then you have the mounting some use screws others use pop rivets and still others a mix of screws or rivets. The holes are not quit the same on the metal plate (let alone the holes in the case). Then the last piece is the ribbon cable/s the given series are different and their mountings to the logic board are different.
I do think Apple is moving that way as there seems to be fewer versions now, but we're talking about the new Retina modes not the older units.