New EMS differential pinion nut.   This is a locking nut.  This is for the 85-91 7.75" (7 3/4") 9-bolt differentials, also known as the Australian or Borg Warner rear as well as the 04-06 GTO M80 rear.  Fits the following:

85-91 Camaro w/ BW 9-bolt differential
85-91 Firebird/Trans Am w/ BW 9-bolt differential
04-06 GTO w/ M80 Differential

NOTE : This does not fit 10-bolt nor 12-bolts.