Section 25. Every person appointed as a registrar or assistant registrar who is not regularly employed by the registrars shall be a voter of the city or town where he is appointed. No person appointed as a registrar or assistant registrar shall hold an office in the city or town for which he is appointed either by election or by direct appointment of the mayor or of the board of selectmen or of a city manager or town manager or hold an office by election or appointment under the government of the United States or of the commonwealth or of the city or town, except as a justice of the peace, notary public or officer of the state militia; provided, however, that in a town with a population of under two thousand residents, a person may be appointed a registrar or assistant registrar who holds an office in the town for which he is directly appointed by the board of selectmen or town manager, or who holds an office by appointment under the government of the United States or of the commonwealth. The acceptance by a registrar or assistant registrar of any such office shall vacate his appointment as registrar or assistant registrar. Nothing in this section shall prohibit a registrar or assistant registrar from becoming a candidate for election and serving as a member of a home rule charter commission or as a town meeting member.