Section 58A. The hours of duty of the permanent members of the uniformed fire fighting force in every city in which this section is accepted by the mayor and by the city council and in every town in which it is accepted by vote of the town at an annual town meeting shall be so established by the fire commissioner, board of fire commissioners, chief engineer, board of engineers, or other officers having charge of fire fighting that the average weekly hours of duty in any year, other than hours during which such members may be summoned and kept on duty because of conflagrations, shall not exceed seventy in number; provided that any such city by ordinance and any such town by by-law may establish such weekly hours of duty at less than seventy. Sections fifty-six, fifty-seven and fifty-nine shall not apply to the permanent members of the uniformed fire fighting force in any such city or town.