Section 123A. In a district, the prudential committee, if any, otherwise the commissioners, may in writing appoint an assistant treasurer who may be an employee of the district, and an assistant clerk of the district. Such assistants shall be sworn to the faithful performance of their duties and a record shall be made of any such appointment and oath. They shall be citizens of the United States and residents of the commonwealth, and shall give bond annually for the faithful performance of their duties in a form approved, and in an amount determined by the commissioner of revenue. Unless a temporary treasurer or a temporary clerk has been appointed in accordance with law, the assistant treasurer or the assistant clerk, as the case may be, may, in the absence of the treasurer or clerk, as the case may be, perform the duties of such office and in the performance of such duties shall have the powers and be subject to the requirements and penalties applicable thereto.