That letter states that they consider it a Title 1 firearm (ex rifle, pistol, shotgun ) not a machine gun. I'm not a lawyer, or legal expert! As of now you can manufacture a title 1 firearm for your own personal use, without a criminal background check, FFL/ manufactures license, or serialization. Examples are; up to 80% machined receivers, 3D printed firearms, homemade, or improvised firearms as long as they are not for commercial sale(so called ghost guns). The devil may be in the details!