The author remixed this model.
This is a mod to the middle support.
The original didn't have a whole for the top center screw. The plexiglass has a hole but the piece didn't.
The hole is threaded M5 because you can't really reach to put a nut on the end. It works good and fixes a very minor oversight. I just didn't like having a hole in the plexiglass with no screw or plug in it.