Here is some info that may help. I know your's is later than this but it's the idea of where to look. After seeing these links I did remember that the it could be stamped on the tube and not a machined area. I had a 67 12 bolt rear in my truck for years and that IS where it was....ahhh the mind is going.
http://chevellestuff.net/qd/generic_rear_axle_info.htm http://stevescamaroparts.blogspot.co...camaro_11.html
Brian