Income derived from the operation of aircraft or a ship or ships by a corporation organized under the laws of a foreign country shall not be included in gross income, and shall be exempt from the taxes imposed by this part if:
(a)
The aircraft are registered or the ships are documented under the laws of the foreign country;
(b)
The income of the corporation is exempt from national income taxes by reason of a treaty or agreement between such foreign country and the United States which provides for an equivalent exemption to corporations organized in the United States; and
(c)
Units of government (other than at the national level) within such foreign country do not impose a tax upon corporations organized in the United States with respect to income derived from the operation of aircraft registered or ships documented under the laws of the United States.