How can I take apart the pictured bed frame joint? It doesn't have visible screw heads on the outside of the footboard (image 1), and the recess in the runner that contains he screw is covered by some kind of lock (image 2). 

I tried turning the hexagonal opening with an Allen key, but it doesn't move for more than half a turn.![enter image description here](https://i.stack.imgur.com/KpB4G.jpg)![enter image description here](https://i.stack.imgur.com/31rm0.jpg)