How about trying it a different way? Do a title search with carfax and call the PO's to ask them what the donor car/truck was.

If that doesn't work and all else fails, since you are probably curious because you need replacement parts, just pull the offending seal/brakeshoe, spring, etc... and take it to NAPA- they can cross the parts using the numbers stamped on them and might also be able to figure out the donor using the same info.

12 bolts weren't used for too long. There are some 'faux 12 bolts' which have 12 bolt covers but 10bolt ring and pinion gears inside them.

12 bolts came in cars and trucks and there is a slight difference between the two types.

There are also three types of 12 bolt posi's- two with carrier's that have clutches and one that is straight gear driven, if I'm not mistaken.

Good luck!

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi