Just to clear things up... I think that the white markings in question are UID markings - Unique IDentifcation labels that are mounted to each weapon nowadays
(see attached article)
https://www.dvidshub.net/image/5977571/us-marines-display-marksmanship-skills-during-competition
Looks like a QR code.
That makes sense.
You're right, the photo FTA blown up shows it, nice job.