Notwithstanding the provisions of section 13-151 of this chapter, any person becoming a member who was not previously a member or who during his last previous membership in the retirement system contributed on the basis of a minimum retirement age of fifty-five, may elect, prior to the certification of his or her rate of contribution; and any member may elect, prior to the first day of October, nineteen hundred forty-nine, to contribute on the basis of a minimum retirement age of fifty-five by a written election duly executed and acknowledged and filed with the board. The minimum age of retirement for such member so electing shall be fifty-five years, and all contributions and benefits payable by or on account of such member shall be computed on the basis of such minimum retirement age. The method of computation and deductions prescribed by this section shall be appropriately modified in the case of a member for whom a rate is otherwise fixed pursuant to section 13-152 of this chapter.