The Civil Service Commission shall consist of three electors of the City, not holding other Municipal office, to be appointed by the Mayor for a term of six years except that, of the three appointed for the terms beginning January 1, 1952, one shall be appointed for a term of two years, one for a term of four years, and one for a term of six years. A vacancy occurring during the term of any member of the Civil Service Commission shall be filled for the unexpired term in the manner authorized for an original appointment.