MCMASTER 3/8-16 SS LOCK NUT

The key is the thread size. This one is made for standard 3/8-16 threads, so it matches a 3/8-16 bolt or stud.

Fully stainless steel construction, not a plated or coated finish. That matters a lot in wet or corrosive environments where a surface treatment would eventually wear through and leave bare metal exposed. McMaster-Carr makes these to hold up in demanding industrial conditions, so you're getting real corrosion resistance throughout the material, not just on the outside.