This is the 5/16" x 26tpi Half Nuts, as fitted to oil pressure adjuster bolt - which does not protrude far enough to allow a full nut, so it is important to fit this special nut.
See the main listing for Iten 0243 to see how the oil adjuster bolt works, but it is always worth keeping a couple of these half nuts spare in your toolbox.
These are in stainless steel and sold as a Pack of 3