The board, upon petition of any party and upon cause shown, at any time before the court of common pleas of any county of this Commonwealth, to which an appeal has been taken under the provisions of section four hundred and twenty-seven of this article,¹ shall have taken final action thereon, may grant a rehearing of any petition upon which the board has made an award or disallowance of compensation or other order or ruling, or upon which the board has sustained or reversed any action of a referee; but such rehearing shall not be granted more than eighteen months after the board has made such award, disallowance, or order or ruling, or has sustained or reversed any action of the referee. If the board shall grant a rehearing of any petition from the board's action on which an appeal has been taken to and is pending in the court of common pleas under the provisions of section four hundred and twenty-seven of this article, the board shall file in such court a certified copy of its order granting such rehearing, and it shall thereupon be the duty of such court to cause the record of the case to be remitted to the board: Provided, however, That nothing contained in this section shall limit or restrict the right of the board, or a referee designated by the board, to review, modify, set aside, reinstate, suspend, or terminate, an original or supplemental agreement, or an award in accordance with the provisions of section four hundred thirteen of this article.²