A final receipt, given by an employe or dependent entitled to compensation under a compensation agreement or award, shall be prima facie evidence of the termination of the employer's liability to pay compensation under such agreement or award: Provided, however, That the board, or a referee designated by the board, may, at any time within two years from the date to which payments have been made, set aside a final receipt, upon petition filed with the board, if it be conclusively proved that all disability due to the occupational disease in fact had not terminated. Where, however, a person is receiving benefits pursuant to the act of June 28, 1935 (P.L. 477, No. 193), referred to as the Heart and Lung Act,¹ the two-year period within which the board or a referee designated by the board may set aside a final receipt upon petition filed with the board, shall not begin to run until the expiration of receipt of benefits under the Heart and Lung Act.