These are considered 10-Bolt rears, Olds & Buick also had this oddball rear. There are very few parts available for these.
Source: Car Craft Magazine - August 1984
Not included in the chart above is the heavy-duty 10-bolt towing package offered by Oldsmobile, which actually has a 10-bolt gear assembly installed in a 12-bolt housing. The ring gear on this setup measures 8.3".
From this link>> http://www.oldsmobility.com/old/10bolt-tech.htm
May try searching for Pontiac 12-bolt or 10-bolt for more Info.
I also read these weren't installed in Goats untill 1970.