WHAT DOES BCM LOWER RECEIVER MEAN?

bcn complete lowerSeemed it in excess of so many times and can't find the blem. Everytime I believe I uncover a little something off, it is just oil smudges. Very best guess at this point is scuffs on castle nut, or the two tiny silver spots within the flare in the magwell. 1mm dots at most. BCM QC will not mess all over It appears.Electroless nick