In any town or borough which has established a police reserve fund under the provisions of this part, the powers and duties vested in and imposed upon the mayor or common council of a city shall, in the case of a town, be vested in and imposed upon the board of selectmen thereof, and, in the case of a borough, be vested in and imposed upon the warden or burgesses thereof, except that, in the case of a borough or a town, all appropriations shall be made by the body authorized to make appropriations therein.