9799100 was the p/n for the center casting used in the '70-'72 Pontiac A body Rear Axle Tubes & Diff Housing Assembly.
I would decode J151 as October 15, 1971 as the date the part was cast.
Since '72 model year production had started by then, would indicate it was originally installed in a '72 Pontiac A body. The stamped axle code would tell you whether it was assembled with open diff or locking diff and what gear ratio. Of course, you could learn all that by removing the cover.
I'm not aware of any other usage for that casting, however some earlier Pontiac produced center castings were used in rear axle assemblies for other GM Divs. (in particular Olds), so not necessarily Pontiac only. The Manifest Code stamping on the axle tube would confirm if it was a Pontiac specific assembly.
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