Help me find my Rearend

[70GMuttC]

I just started tearing my 1971 K10 project apart. I opened the rear diff to start cleaning it up and found it wasn't opened, but had a posi or locker of some kind. We started looking closer and found the axle perches are 2" wide (from a car?) instead of 2 1/2" and have some really bad welds on them as well as the shock mounts. But what I really need help with is the gears in the rear, a 12 bolt, but the gears are stamped with a 3/83 manufacture date. So if anyone can help...PLEASE?!?!? Oh yeah the whole gear stamp if anyone can decode it: 1 GM 339635 11 41 3 83. I'll post pictures of the posi lookin thing when I get to work tomarrow.

Thanks,
Jason