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  As enacted in 1938, the FLSA, 29 U.S.C. § 201 et seq., required employers engaged in the production of goods for commerce to pay their employees a minimum wage of 'not less than 25 cents an hour,' § 6(a)(1), 52 Stat. 1062, and prohibited the employment of any person for workweeks  in excess of 40 hours after the second year following the legislation 'unless such employee receives compensation for his employment in excess of [40] hours . . . at a rate not less than one and one-half times the regular rate at which he is employed,' id., § 7(a)(3), ...