12 bolt olds rear
You have a full size 12 bolt olds rear diff. 1967-1970ish deltas 88 98 had a 12 bolt FULL size 'O' type 12 bolt rear. This rear came with 12 bolt symmetrical cover and 12 bolt ring gear .
The 12 bolt mid size or A-body GM 'O' 12 bolt with a symmetrical cover has 12 bolts on the cover and 10 bolts on the ring gear. These were in the olds cutlas, 442 AMERICAN made cars only 1967 -1970. Canadian car received the 12 bolt chevy rear and was open non posi diff.
then in 1971 1972 the rear was a 10 bolt 8.5 corperate just like the all the 70's and 80's cars and 1/2 ton trucks...ONLY they came wioth bolt in axles.
1973 was the switch to a new body style so that rear will not bolt into the 1972 and older body style.
So I am sure you have a FULL size 12 bolt 'O' type rear in your hands there. If I had pictures of the housing cover and or posi unit I could ID it for sure. Jim