The City Planning and Zoning Commission shall consist of five 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 five appointed for the terms beginning January 1, 1952, two shall be appointed for a term of two years, two 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 City Planning and Zoning Commission shall be filled for the unexpired term in the manner authorized for an original appointment.