I have replaced the pinion seal in my 1966 Skylark GS three times in the last year. The whole story is this. When I bought the car it had an open rear. I purchased a positraction unit and had it installed by a qualified mechanic. At his suggestion, even though there was no leakage at the original pinion seal, he installed a new one, NAPA #16500. That seal leaked all most immediately. The Auto Zone replacement, Timkin #2043 was obviously to big. Some how, NAPA #15305 was suggested and installed. It has just begun leaking after about 500 miles of usage. Another parts supplier lists National #8611N as the correct replacement. In looking at it, it appears to be identical to the NAPA #15305. I don't want to go to the trouble of installing this one if it will only last another 500 miles. I was able to obtain an original GM #1387449 which is listed in my parts book as the correct seal. It "appears" fine. At the outer edge of the lip, it measures 1.63", while the NAPA #15305 and NATIONAL #8611N both measure 1.50". Any suggestions/help is greatly appreciated.