'70 12 bolt 3.73 Posi
 

I wonder if this is an original 3.73 Eaton or if someone put those parts in it.
https://lasvegas.craigslist.org/pts/...442145788.html

Pretty sure 3.73:1 was not an available ratio on a '70 SS. Or any '70 GM A-body for that matter.

Correct.
But I would guess that 12 bolt housing came out of a '70 SS Chevelle originally,and someone swapped out the factory gears for 3.73,is what he's insinuating.