(a) Section 18.1 shall not apply:
(1) When a statute which meets the requirements of 5 U.S.C. 3347(a) prescribes another means for authorizing an officer or employee to perform the functions and duties of a PAS Office in the Department temporarily in an acting capacity; and
(2) To the office of a member of the Internal Revenue Service Oversight Board.
(b) The Inspector General of the Department of the Treasury shall determine any arrangements for the temporary performance of the functions and duties of the Inspector General of the Department of the Treasury when that office is vacant.
(c) The Treasury Inspector General for Tax Administration shall determine any arrangements for the temporary performance of the functions and duties of the Treasury Inspector General for Tax Administration when that office is vacant.