(a) For purposes of the regulations in this part, the term “registered” when used in reference to a highway motor vehicle means -
(1) Registered under the law of any State or Territory of the United States, the District of Columbia, or contiguous foreign country, or
(2) Required to be registered under the law of any State or Territory of the United States or contiguous foreign country in which such highway motor vehicle is operated or situated or, in case the vehicle is operated or situated in the District of Columbia, under the law of the District of Columbia.
Any highway motor vehicle which is operated under a dealer's tag, license, or permit is considered to be registered in the name of such dealer. A highway motor vehicle is not considered to be registered solely by reason of the fact that there has been issued a special permit for operation of the vehicle at particular times and under specified conditions.
(b) Any highway motor vehicle which, at any time in the taxable period, is registered both in the name of the owner of the vehicle and in the name of any other person, is considered, for purposes of the regulations in this part, to be registered, at such time, solely in the name of the owner of the vehicle.