The liability under this chapter of a contractor, subcontractor, or subordinate contractor engaged in public work under subparagraphs (3) and (4), subdivision (a) of this section, and the conditions set forth therein, shall become applicable to contracts and subcontracts heretofore entered into but not completed at August 16, 1941, and the liability under this chapter of a contractor, subcontractor, or subordinate contractor engaged in performance of contracts, subcontracts, or subordinate contracts specified in subparagraph (5), subdivision (a) of this section, and the conditions set forth therein, shall hereafter be applicable to the remaining terms of such contracts, subcontracts, and subordinate contracts entered into prior to but not completed on the date of enactment of any successor Act to the Mutual Security Act of 1954, as amended, and contracting officers of the United States are authorized to make such modifications and amendments of existing contracts as may be necessary to bring such contracts into conformity with the provisions of this chapter. No right shall arise in any employee or his dependent under subparagraphs (3) and (4) of subdivision (a) of this section, prior to two months after the approval of this chapter. Upon the recommendation of the head of any department or other agency of the United States, the Secretary of Labor, in the exercise of his discretion, may waive the application of this section with respect to any contract, subcontract, contract, or subordinate contract, work location under such contracts, or classification of employees. Upon recommendation of any employer referred to in paragraph (6) of subsection (a) of this section, the Secretary of Labor may waive the application of this section to any employee or class of employees of such employer, or to any place of employment of such an employee or class of employees.