From the good folks at The Camaro Research Group
http://www.camaros.org/diffs69.shtml
If the car was to early to have an "X" code (Norwood cars only) then the big thing would be checking that the numbers match on the engine pad/tranny. Is there any paperwork or Protect-O-Plate (POP) with the car? Also check the rear end codes on the passengers axle tube facing foward.
Plenty more help over on the Team Camaro site as well.
http://www.camaros.net/forums/
You can post the numbers from the car here or at the TC site and someone will help out decoding them for you.