To require each fireman, including those on vacation and on leave of absence to file a statement, in such form as the board directs, concerning service rendered prior to the effective date, from which the board shall make a determination of the length of such service; to determine, from such information as shall be available, the period of service rendered prior to the effective date by any fireman who fails to file such a statement.
     The determination by the board shall be conclusive as to any such period of service unless the board reconsiders any case within 2 years from the date of the determination and changes the determination.