So now that my car's front end is happy, I just needed to laydown a burnout in celebration. Well, despite what I was told, it ain't an "anti-slip" diff.
It has 12 bolts holding the cover so I think that means a 10 bolt carrier inside. Will look for numbers stamped on the diff and see if I can figure out what it is and if it is reasonable dollars to convert - with 1 wheel drive, it makes no sense to do anything for horse power if I can't use it. There's more confusing mumbo-jumbo talk online about differentials that any other part of the car. Time to study.


(and I'm not touching the AC leak issue in this heat wave)

 

A 1970 Cutlass should have an O-Type rear, which as you said has a 12 bolt cover and a 10 bolt ring gear. There are different series carriers for different gear ratios, with one for 2.56 and 2.78, one for 3.08 and 3.23, and another for 3.42, 3.91, 4.33, etc. You may actually have an anti-spin and the clutches are worn out; gotta remove the cover to verify if it's an open carrier or a posi.

Any idea what gear ratio you have? Some carriers are more plentiful than others.

 

There is a code stamped on axle tube.Should be on passenger side.You may need a wire brush to find it or scotch bright.