[Mr70]

I would magine those are Harrison Batch/shift numbers for quality control.
I have some here:
BO w/IA & XJ.number 4 underneath.70 L78 Manual
BJ w/IA & XI.number 16 underneath.70 L78 Auto.
RP w/IA & AG.number 82 underneath.
AC w/IA & XG.number 4 underneath.
VB w/IA & CF.number 5 underneath.
DX w/IA & AA.number 8 underneath.
VI w/IA & CF.number 58 underneath.
I know they are repopin these tags for Camaros,& I hope the other cars are coming soon too...