Its funny that they wouldnt have drilled for it if the zkk's originally came with the third hole, you might want to give them a shout if you are going to order one.
I ordered another stock with this one and the total was just around 320.00 delivered for the pair ( the other was an applejack laminate rimfire hunter for my cz 452 varmint 92.00)
The stock on the 550 is a nutmeg laminate prarie hunter with a limbsaver pad and the fleur de lis pattern scale which will provide good grip when its wet.