[Jake1074]

Are these axle casting numbers of any help identifying the rear differential?

Passenger side next to fill plug - 3896838
Drivers side - GM 66

I tried to visualize the top of the right side for stampings but didn't see any may be too dirty.

It's in a '67 that has been converted to 5 lug - open rear. The SPID was a 3.73 posi rear, but apparently no longer.