The Civil Service Commission shall consist of three (3) electors of the City, not holding other municipal office or appointment, to be appointed by the Mayor, for terms of six (6) years each, except that of the three appointed for the term beginning January 1, 1959, 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.