The board, if a petition is directed to be heard by it or by one or more of its members, or the referee to whom a petition is assigned for hearing may subpoena witnesses, order the production of books and other writings, and hear evidence, and shall make, in writing and as soon as may be after the conclusion of the hearing, such findings of fact, conclusions of law, and award or disallowance of compensation, or other order, as the petition and answers and the evidence produced before it or him and the provisions of this act shall, in its or his judgment, require. The findings of fact made by the board in any petition heard by it or by one or more of its members or upon a hearing de novo shall be final, except as hereinafter provided, and the findings of fact made by a referee to whom a petition has been assigned or any question of fact has been referred under the provisions of section four hundred and nineteen¹ shall be final, unless an appeal is taken as provided in this act, or unless the board shall, under the provisions of sections four hundred and twenty-five or four hundred and twenty-six of this article² grant a hearing de novo or a rehearing.