Special, 5/16-UNF all-metal locking nut to be used on drive shaft yoke u-bolts. This application cannot use normal Nyloc-type nuts due to continuous high heat from the exhaust system.
This type of nut is commonly called a "Stover" nut in the USA, but a nerdy bit of information is that these are called "Philidas" nuts in England, based on the company that made a similar design there. We don't supply a particular brand, but they will be all-metal.