According to the rcinfos article from Nuremberg '06 from which I took the pictures, the top plate also had pins to position itself. I believe we can see one here :
http://www.rcinfos.com/2006/2006_02_...0/P1010335.jpg
Again, they didn't keep it. And I fail to see how it would bring a more precise/reliable alignment than a well manufactured top plate with countersunk holes, or like Xray does, button head screws with a tiny cone under the head.