New National Bearings differential pinion seal.   This is for the 8.5" 10-bolt differentials.  Fits the following:

70-81 Camaro w/ 8.5" 10-bolt differential
70-81 Firebird/Trans Am w/ 8.5" 10-bolt differential

NOTE : This does not fit 8.2" 10-bolt differentials found on some 70-72 cars, you will need to measure the ring gear if you are not sure which you have.