§ 25. "That the extent and incidence of the national defence contribution (as respects all chargeable accounting periods beginning on or after the first day of April, nineteen hundred and thirty-nine, and so much of any chargeable accounting period beginning before that date as falls on or after that date) shall be varied so as to give effect to amendments as to the computation of profits and losses, interconnected companies, and the relation of the national defence contribution to excess profits tax."