FWIW, a few years back I sold a 12 Bolt Posi out of a '67 330/310 Cutlass.. I looked online for casting info etc as there was a big K or M or some letter cast into it but never found any more than the code/ratio either?. I never cracked it open as it sold quickly but I wanted to as I'd read Type O's had 10 bolt rings even though a 12 bolt cover?.
Also from what I understand, the 'O' rears had outboard axle retainers not c-clips like Chevys and were preferred by racers because of this...also some guys converted these axles and/or retainers into the Chevy rears?.
~ Pete
