if he pleads and proves that the act or omission complained of was in good faith in conformity with and in reliance on any administrative regulation, order, ruling, approval, or interpretation, of any agency of the United States, or any administrative practice or enforcement policy of any such agency with respect to the class of employers to which he belonged. Such a defense, if established, shall be a bar to the action or proceeding, notwithstanding that after such act or omission, such administrative regulation, order, ruling, approval, interpretation, practice, or enforcement policy is modified or rescinded or is determined by judicial authority to be invalid or of no legal effect.