Section 74. Sections sixty-nine to seventy-five, inclusive, shall apply to all laborers, workmen and mechanics in the service of the commonwealth or of such county, city, town or district under any employment or contract of hire, expressed or implied, oral or written, including those employed in work done in performance of governmental duties as well as those employed in municipal enterprises conducted for gain or profit. Said sections shall not apply to inmates of institutions performing labor under sections forty-eight to seventy-seven, inclusive, of chapter one hundred and twenty-seven. For the purposes of sections sixty-nine to seventy-five, inclusive, all laborers, workmen and mechanics paid by the commonwealth, but serving under boards or commissions exercising powers within defined districts, shall be deemed to be in the service of the commonwealth. For the purposes of sections sixty-nine to seventy-five, inclusive, all employees of welfare districts, organized under the provisions of section forty-four of chapter one hundred and seventeen, shall be deemed to be employees of such town as the district welfare committee shall determine, and if such town has provided for the payment of compensation required by this chapter, the said district welfare committee shall apportion the expenses of providing such compensation among the towns comprising the said welfare district.