Section 21. There shall be a board to be known as the commissioners on firemen’s relief serving in the department and consisting of the state treasurer, ex officio, two members appointed by the governor, with the advice and consent of the council, and two members appointed by the Massachusetts state firemen’s association. Upon the expiration of the term of office of an appointive member, his successor shall be appointed as aforesaid for two years. Any vacancy existing shall be filled for the unexpired term in the manner of the original appointment.