The way I've tested for a posi rear end is to jack it up til the rear wheels are off the ground (tranny in neutral) and turn one of the rear wheels. If the other one turns in the same direction, it's a posi. If they turn in opposite directions, it's open.
I know this works on 63 and later 10 and 12 bolt and GM corporate 7 5/8 inch rear ends. I'm not sure about earlier stuff.
Ray
Chevy Asylum
