Section 13-247. Retirement; selection of either twenty or twenty-five years of city-service

a. Any person becoming a member who was not  previously  a  member  or  who  during  his  or  her last previous membership in the
  pension fund contributed on the basis of a minimum period of  retirement
  of  twenty  years of city-service, may elect, prior to the certification
  of his or her rate of contribution, to contribute  on  the  basis  of  a
  minimum  retirement period of twenty years of city-service, by a written
  election duly executed and acknowledged and filed with  the  board.  The
  minimum period of retirement for such member so electing shall be twenty
  years  of city-service, and all contributions and benefits payable by or
  on account of any such member shall be computed on  the  basis  of  such
  minimum retirement period.
    b. Any person becoming a member who was not previously a member or who
  during  his  or  her  last  previous  membership  in  the  pension  fund
  contributed  on  the  basis  of  a  minimum  period  of  retirement   of
  twenty-five  years of city-service, may elect, prior to certification of
  his or her rate of contribution, to contribute on the basis of a minimum
  retirement period of twenty-five years  of  city-service  by  a  written
  election  duly  executed  and acknowledged and filed with the board. The
  minimum period of retirement for  such  members  so  electing  shall  be
  twenty-five  years  of  city-service, and all contributions and benefits
  payable by or on account of any such member shall  be  computed  on  the
  basis of such minimum retirement period.