. . . . "In the latest of the foregoing cases, Connally v. General Construction Co., 269 U. S. 385, 291, 46 S. Ct. 126, 70 L. Ed. 322, the validity of a statute of Oklahoma providing that not more than the current rate of per diem wages in the locality where the work was performed should be paid to laborers, workmen, mechanics, prison guards, janitors in public institutions, or other persons so employed by" .