It is a new rule/law, all new significant component, pressure bearing parts of a firearm need to be numbered now. The police system is not set up to receive these from what I understand. It wants one serial number per gun, now it will need 3 or 4 logged for each gun. Breach,/barrel, bolt, receiver and uppers and lowers for ar types.
I have seen a post from Lantac who make AR type guns where they are getting everything numbered up for a batch of new guns.
Edit.
https://m.facebook.com/story.php?story_fbid=3438208749577957&id=148402105225321
That should be a link to their facebook post about it.
Matt