All collective bargaining clauses which, in the judgment of the federal or state contracting agency administering the contract, preclude, prohibit or in any way discourage employers or groups of employers from engaging in any federal, state or on-the-job apprentice training program approved by any federal or state agency so empowered shall be void and unenforceable. This section shall not apply to any collective bargaining agreement in effect on July 1, 1969, for the duration of such agreement.