Point #3 under registration is for those guns that the State doesn't know about.The new bill specifically mentions "registered" which is a specific sub-function of the eFA-10 Transaction Portal.
Note that when a seller (private or dealer) enters a transfer they are recording a transfer NOT registering a gun.
Registration is a separate function within the portal.
So the number of 1994->2016 ARs that are "registered" in the database is highly likely to be much less than the number of AR transfers in the database.
Massachusetts courts are going to interpret the language in a light favorable to the state - in this case I would posit that means if you didn't go in and add your ARs through the register option then you may not be grandfathered if the AG wants to push the issue
If you did an FA10 when purchasing it, the State knows.
If you did an FA10 when building it, the State knows.
Therefore all the guns that have gone through the FA10 process, either someone entering as part of a private sale, FFL sale or assembling the gun, are "registered".