WHAT DOES BCM LOWER MEAN?

bcn complete lowerSeemed it around so repeatedly and can't discover the blem. Everytime I think I locate anything off, it is simply oil smudges. Very best guess at this time is scuffs on castle nut, or the two tiny silver spots from the flare from the magwell. 1mm dots at most. BCM QC isn't going to mess about it seems.To my delight I found that im