[Mr70]

It's an assembly line internal tracking code.
It was assigned to a vehicle by Fisher Body,and for their useage only.GM bought Fisher Body works beginning with the 1970 production,and internal data was merged and downsized or just carelously deleted.
In years prior & on 1970 Oshawa,Canada built vehicles,Chevrolet listed some of the vehicles RPO codes there,so some have assumed that it must be an option description of sorts.As of today,no records or files have ever been found to prove this theory.
It's believed to have no bearing on what the vehicle was to be equipped or built as.And it had no specific use or substance once the vehicle left the assembly line,and to date it's been undecipherable.