Nice catch. It’s basically a capped nut machined from aluminium with a fairly deep thread, and a nylon rod that makes it a lock nut.
It’s designed to cover the bearing and look somewhat like a centerlock car wheel nut.
We also developed a fibreglass nylon tool for installing it, which fits on to a normal T tool or socket. This prevents the anodized surface from being scratched. And I believe we are the first to come up with this solution.
