Basically, it’s an expansion plug with a plastic disc mounted to it.
The rubber “puck” is pressed inside the centre of the wheel. There’s a little lip in the centre where the wheel mates with the hub on the suspension. The rubber is very slightly looser than that lip. Tightening the centre-nut on that bolt (I wish it was a machine screw..it’s just a carriage bolt lol), tightens everything which compresses the rubber and locks against the inner lip on the wheel.
In other words, it’s a fancier form of an expansion plug, with a plastic disc attached at the opposing end of the bolt.
The nut is external, while the square shoulder of the carriage bolt locks against an aluminum washer closest to the hub. It works great. In my previous post, you can see the two different “stacks”; the wheel disc goes behind the aluminum washer closest to the nut.
Hoping I’m in an innovator, haha.