The Heat-Treated Castle Nut screws onto the AR15/M4 lower receiver extension (buffer tube) and keeps the receiver extension and lower receiver end plate secured in place.  The large notches allow for positive engagement with a castle nut wrench for proper installation and torque.  The smaller notches provide a location for the end user to stake the castle nut to prevent accidental loosening.

Finished in a Mil-spec manganese phosphate.