(b) Exceptions.— A person may operate an aircraft in the United States that is not registered
(1) when authorized under section
40103 (d) or
41703 of this title;
(2) when it is an aircraft of the national defense forces of the United States and is identified in a way satisfactory to the Administrator of the Federal Aviation Administration; and
(3) for a reasonable period of time after a transfer of ownership, under regulations prescribed by the Administrator.