Received this non-answer answer from the AG's office this morning:
"Mr. XXXXXX: We have discussed this issue with the ATF, but we don’t control the licensing choices they make.
In general, our position has been that you can build out a non-compliant receiver and register your rifle, provided you retain evidence that the receiver was acquired before July 20, 2016.
Perhaps the ATF can be more specific on your application at this time.
Best,
Gary Klein
Senior Trial Counsel
Public Protection and Advocacy
Office of Attorney General Maura Healey
"