Maybe the marks represent the worker who went back and finished or gauged the cock on opening ramp, and did final polishing/fitting of the bolt lug or drilled the gas escape hole checked bolt face etc.
I'm starting to think the marks didn't have anything to do with headspace. The letters numbers and marks might just reflect payroll or tool room or equipment logs. Maybe even for machine ID in case a tool became out of spec before the rifles made it to another station during manufacture so it could be identified for maintenance or adjustment.
Just a guess on my behalf, they seem to be too randomly done.