Interesting that the MPC tag diagram lists the three digit number in the lower left hand corner as the "plant code". This could be interpreted as "the code identifying where plant where the axle was built" or "an code used for quick identification of the rear axle at the assembly plant" or ...
Is there any information available in the Marti report? If I recall Marti reproduces these tags, so might be able to validate the number. I have not used those services so can't say for sure.
Since Jeff's example with an 8J date links the WES-M assy to identification number 936, and you have an 8A date...you might have been given the answer. Something closer to confirm that there was no change in between would be nice.
Jeff, it is my understanding that the axles were shipped out of the Sterling (Heights, Michigan) axle plant as complete assemblies, so would think that the tag was stamped and affixed at the axle plant. That would mean that a 936 would be the same across all assembly plants. Any comment or correction to this?