The governing body of any political subdivision, municipal corporation, or district, and any public officer or person charged with the letting of contracts for (1) the construction, alteration, or repair of public works or (2) for the purchasing of materials for public use, shall let such contracts only to persons who agree to use or supply only such unmanufactured materials as have been produced in the United States, and only such manufactured materials as have been manufactured in the United States, substantially all from materials produced in the United States.