These are door handles for 2 door that meet each other in the closed position :-)
But of course they can also be used single.
The beam in the picture where the “hook” grasps on, is 20x20mm
I think the picture makes it all more clear.
Part1 and part2 are connected by slotting in some peace of hard wire in the back of part1.And then close the gap with part4. Of course these peaces of wire must be at the correct length. The ideal length should be 21mm. I just cut them from electrical wire (2,5mm²) Part 5 is just glued to Part2.