The grinding you see is where the early A body spring perches were removed, They springs mount in front of the axle tubes not on top like a G body.
Looking at the side of the case in front of the axle tubes there are ribs.
1 rib running from the front at about the center line of the pinion to the center of the axle tube is a Buick.
2 ribs one same as above with another angled from above the top of the axle tube running down to the other rib at the front is a Pontiac.
2 ribs running parallel at the top and bottom of the axle tubes is an Olds or Chevy. (Some Chevys did have that double cut cover.) To tell the difference the Olds rear end should be bearing retainer axles, the Chevy will be C clip retained axles.
I think the 61" width says it is a 68-72.
There are numbers stamped on the axle tube to identify ratio, month, day and posi or not. Buick, Pontiac and Chevy are usually on the passenger side tube on the front. Olds are usually on the drivers side tube on the back. It might help with more identification.
Formatted like this:
XX XX XX X Late ones might be *** XX XX X
On the triangular gussets on the bottom (Right side one is just right of the tag on the cover.) there are numbers. One is a date code.
