Under regulations prescribed by the Secretary, no tax shall be imposed by section
4481 on the use of any bus which is of the transit type (rather than of the intercity type) by a person who, for the last 3 months of the preceding year (or for such other period as the Secretary may by regulations prescribe for purposes of this subsection), met the 60-percent passenger fare revenue test set forth in section
6421
(b)(2) (as in effect on the day before the date of the enactment of the Energy Tax Act of 1978) as applied to the period prescribed for purposes of this subsection.