The Mayor shall appoint all members of the Commission and shall designate, at the time of appointment, the regular members by class and the alternate members as "Alternate No. 1" and "Alternate No. 2." The terms of the members first appointed under this Part 3 shall be so determined that to the greatest practicable extent, the expiration of the terms shall be distributed, in the case of regular members, evenly over the first four years after their appointment, provided that the initial term of no regular member shall exceed four years and that the initial term of no alternate member shall exceed two years. Thereafter, the term of a regular member shall be four years; and the term of an alternate member shall be two years. A vacancy occurring otherwise than by the expiration of term shall be filled for the unexpired term only. Notwithstanding any other provision herein, the term of any member common to the Historic Preservation Commission and the Planning Board shall be for the term of membership on the Planning Board, and the term of any member common to the Historic Preservation Commission and the Board of Adjustment shall be for the term of membership on the Board of Adjustment.[2]