The Civil Service Commission shall consist of three (3) electors of the City, not holding other Municipal office, to be appointed by the Mayor for a term of six (6) years, except that of the three (3) appointed for the terms beginning January 1, 1952, one shall be appointed for a term of two (2) years, one for a term of four (4) years, and one for a term of six (6) 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.