Section 49A. The auditor, accountant, or officer having similar duties, of a city or town, may in writing appoint, with the approval of the mayor or the selectmen thereof, an assistant, who shall be sworn to the faithful performance of his duties, and a record shall be made of his appointment and oath. Unless a temporary officer is appointed in accordance with law, the assistant may, in the absence of the appointing officer, 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 hereunder may receive a salary from the city or town for services as such.