§ 123. Jurisdiction of board to be continuing. The power and jurisdiction of the board over each case shall be continuing, and it may, from time to time, make such modification or change with respect to former findings, awards, decisions or orders relating thereto, as in its opinion may be just, except that, where the employer has secured the payment of compensation in accordance with the provisions of section fifty of this chapter, no claim for compensation or for death benefits that has been disallowed after a trial on the merits, or that has been otherwise disposed of without an award after the parties in interest have been given due notice of hearing or hearings and opportunity to be heard and for which no determination was made on the merits, shall be reopened after a lapse of seven years from the date of the accident or death. Nor shall any award of compensation or death benefits be made against the special fund provided in section twenty-five-a of this chapter or against an employer or an insurance carrier where application therefor is made after a lapse of eighteen years from the date of the injury or death and also a lapse of eight years from the date of the last payment of compensation.