That is a full size pontiac 10 bolt cover rear with 12 bolt guts 8 7/8 ring gear. Does it have bolt in axles or c-clip axles. If you get me a shot of the backing plates from under the car I can tell you.
It looks to be posi as the tag is there. Best ratios for this rear I think was 3.42 if it is a c-clip type and the bolt in axletype from the late 60's the best ratio that you could get was 3.73 ...
The pictures show the 1971 -1977 12 bolt with 10 bolt cover posi units. The older late 60's units will look about the same except they only have 2 preload springs.
NO AFTERMARKET SUPPORT FOR THESE REARS AT ALL...
No posi unit No gears. what is out there is what you rget. Jim