as far as i know this is a 65 cutlass posi rear diff but can't find any info on the codes i have found on it

M N D or possibly M 1\1 D

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You need to posi pictures of the rear end cover yoke and over whole rear. We need to know what rear your dealing with to look up a code. Thank you, Jim

D is stamped pretty deep I bet it si a 1964 f85 rear 3.55

OR
if it is a 65 then it would go off the M N letter code of 3.08... Hard to say. I would have to say someone goof at the plant with the stamping. Remove the cover and wipe the outer edge of the ring and it should have the tooth count stamped into the ring.

 

here are some more pics, it is the one at the bottom of the pics

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Well it appears to be a 65 rear as the upper control arm bushings are large 2 inch diameter the 64 will be 1" diameter upper control arm bushings. The rear is also a BUICK 10 bolt 8.2 Not the Oldsmobile type.
The Buick 10 bolt was used in all lower HP cars (standard build rear)
Only way your going to tell with this rear is to crack it open and spin the ring wipe the edge until you find the stamping in the edge of it. Jim

 

X2....it's been 50years since this was made. original gears and posi could be long gone/swapped for something else. Best path is to open the cover and look inside.

 

Thanks for the info, I will try and check it out over the holidays