New Pioneer rocker arm nut kit.  This is a high performance kit.  These are locking nuts with a set screw.  You tighten the nut as normal for lash adjustments, then tighten the set screw down to lock the nut, then snug the main nut again.   For performance studs (7/16" thread).  Sold as a set of 16. Fits the following:

67-81 Trans Am w/ Pontiac 265-301-326-350-400-455 Engines w/ 7/16" Studs