"12 bolt rear cover = 12 bolt rear"
It's definitely a 12 bolt cover, but it's like there's this possible oldsmobile exception
"Wouldn't it just be a generic GM 10 or 12 bolt if it was from that period ? , or were there differences between BOP and Chevy ?"
Actually there was a difference between olds and the rest apparently on some of the cars produced during those years.
Here's all the details I could find guys:
Check it out and tell me what you make of it. It's like "c'mon a 12 bolt cover with 28 spline axles and a 8.2" ring gear!?!?!? WTF were you engineering guys thinking!?!?"
I wouldn't worry about it if I didn't need the strength of a 12 bolt.