These 1/4 inch, nickel-plated, standoff hex nuts are the same part we use on all Alembic bass and guitar truss rods. The rods are stainless steel, but the nuts are nickel plated brass - they are designed to be the part that eventually wears out as replacing the nuts is preferable to an expensive repair of replacing stripped out truss rods.
Sold as a set of 2 nuts and 2 washers. If you have a single truss rod, you'll have a spare!
NOTE: Use only these original nuts - if you use a stainless steel one thinking they won't wear out you run the risk of galling, where the nut can adhere to the threads.