Section 39A. The treasurer of a city or town may in writing appoint, with the approval of the mayor or the selectmen thereof, an assistant treasurer who may be an employee in the treasurer's department. The assistant treasurer shall be sworn to the faithful performance of his duties, and a record shall be made of his appointment and oath. The assistant treasurer shall be a citizen of the United States and shall give bond annually for the faithful performance of his duties in a form approved, and in an amount determined, by the commissioner of revenue. Unless a temporary treasurer is appointed in accordance with law, the assistant treasurer may, in the absence of the treasurer, perform his duties and when performing such duties shall have the powers and be subject to the requirements and penalties applicable to him. The person appointed as assistant treasurer may receive a salary from the city or town for service as such.