Members of the fire department, as defined in Section 8.569, who are members of the Retirement System under Sections 8.507, 8.509, or 8.567 of the Charter on the first day of July, 1949, and persons who become members of said department after said date, shall be members of the Retirement System under this Section 8.568 on and after said date, and shall be subject to the following provisions of Section 8.568 and Sections 8.569, 8.570, 8.571, 8.572, 8.573, 8.575, 8.576, 8.577, 8.578, 8.579, 8.580, 8.581 in addition to the provisions contained in Sections 3.670-3.672, 8.500-8.504, 8.506, 8.510 and 8.520 of this Charter notwithstanding the provisions of any other section of the Charter. Members of the said department who are members of the Retirement System under Sections 8.507 or 8.509 of the Charter, on July 1, 1950, however, shall have the option to be exercised in writing, on a form furnished by the Retirement System and to be filed at the office of said system not later than 90 days after said date, of being members of the system under Sections 8.507 or 8.509 instead of Section 8.568, the election under said option to be effective on said date, provided, that members who are absent by reason of service in the armed forces of the United States or by reason of any other service included in Section 8.520 of the Charter, on July 1, 1949, shall have the same option of electing to be members under Sections 8.507 or 8.509, as the case may be, instead of Section 8.568 until 90 days after their return to service in the fire department. On and after said date the persons who affirmatively exercise said option, shall continue to be members of the system under Section 8.507 or 8.509, respectively, and shall not be subject to any of the provisions of Section 8.568.