Hi NM,
The machine gun has a small peg that inserts into the side of the post mounted in the center of the Kubel. Once inserted, it stays in quite well but the center post does not detach from the Kubel. Here's a picture for illustration:
DAK Kubelwagen by
Western Outlaw, on Flickr
And an older shot "in action":
Convoy by
Western Outlaw, on Flickr
It's a great piece and a "must-have" for one's DAK collection. I also like the hitch in the back in case you ever need to pull a small gun or fuel cart (gives me an idea:wink2

.