[Mr70]

I think GM began stamping their service components shortly after the 1967 model year,when their 5/50 power train warranty came out.
Earliest replacement part I've ever seen was a 1968 transmission stamped CT,but others may have seen earlier.
Each plant was assigned its own unique series of unit sequence numbers to stamp on their components.
I recently found a GM catalog showing all of them,but I don't want to list them here,as those who practice to deceive could use them to fool others into believing their homemade re-stamped part is authentic.