Though there is nothing wrong with the factory stock on the WE G39 gas blowback carbine, this kit comes with an upgraded design which is not only foldable but also length adjustable and has an adjustable cheekrest.
While there are kits out there that will allow an AR stock onto a G39, this stock does not disrupt the aesthetic if the G39 as it is not a distinctively AR stock but an entirely new design. It also seems to conform to the shape of the G39 better, much like the stock did on the old XM8.
This kit also comes with a replacement top rail that is almost the same except the built in iron sight are the flip up type. When not in use, they flip down right into the rail and lie flush so they do not protrude above the rail surface at all.