Nyloc Nut with a 3/8″ UNF thread and a 12 mm across-flats hex, in the full-height (thick) form.

A nylon insert set into the crown of the nut deforms over the bolt thread as it is tightened, resisting loosening under vibration. This is a general-purpose self-locking nut used across a range of fastening points, including driveline flange fixings.