Interesting anti-theft nut on a bolt-on bike rack. Being symmetric, I don't think this would resist being turned by large-enough Vise-Grips—but in this application, with the recessed surround, it's probably sufficient.
Deployed on a parking meter, which is fine as-is for u-locks, but wouldn't be useful for chain or cable lockup otherwise. 😎
It's good when the city invests in bicycle infrastructure. 👍