(1) All provisions of this part 13, including membership in the multistate tax commission, shall be effective upon execution of the compact by the governor; except that:
(a) Provisions of articles III, IV, V, VIII, and IX of the compact shall apply to all taxable years beginning on and after July 1, 1968; and
(b) In no case shall the provisions of this part 13 apply to taxable years commencing on or before June 30, 1968.