No person shall transfer or possess a machine gun except:

(a) A transfer to or by, or possession by or under the authority of, the United States, or any department or agency thereof, or a State, or a department, agency, or political subdivision thereof. (See Part 479 of this chapter); or

(b) Any lawful transfer or lawful possession of a machine gun that was lawfully possessed before May 19, 1986 (See Part 479 of this chapter).