(f) No employer shall be deemed to have violated subsection (a) by employing any employee for a workweek in excess of the maximum workweek applicable to such employee under subsection (a) if such employee is employed pursuant to a bona fide individual contract, or pursuant to an agreement made as a result of collective bargaining by representatives of employees, if the duties of such employee necessitate irregular hours of work, and the contract or agreement (1) specifies a regular rate of pay of not less than the minimum hourly rate provided in subsection (a) or (b) of section 6 (whichever may be applicable) and compensation at not less than one and one-half times such rate for all hours worked in excess of such maximum workweek, and (2) provides a weekly guaranty of pay for not more than 60 hours based on the rates so specified.