The Kübel came in a variety of versions for different purposes, so all cars didnt have the four carbine holders. Out of my memory I think the three seat funkwagen had a holder for two carbines - but will have to check it up.
I have a radio kubel. Holders for butts are the original positions and the rail is original. Exact copies of the clamps have been attached to the welded clamp holder. Oddly the carbines when fitted stand at an angle sloping backwards not vertical. Has anybody else found this? Perhaps it is correct.