Whenever the duties imposed by this part upon selectmen are imposed by the charter of any municipality upon any other officer or officers, the term “selectmen”, as used herein, shall be construed to apply to such other officer or officers, who shall be vested with all the powers and duties and shall be subject to all the obligations imposed by this chapter upon such selectmen. In any municipality where by charter the duties of selectmen are limited to the admission of electors and are not imposed by charter upon any other officer or officers, the term “selectmen”, as used herein, shall apply to the registrars of voters of such municipality, who shall be vested with all the powers and duties and shall be subject to all the obligations imposed by this part upon such selectmen.