Maybe you're right. I was up pretty late last night looking for it. I might have read it wrong.
How about the 12 bolt rear? Can anyone verify that this was only on the '70 models and not earlier?
In the meanwhile, I did find that there were quite a few axles from '68 to '70 with no code.
See here-
http://www.teufert.net/pontiac/rears.htm