Proof of consent, etc., of Board or Chairman or officer or servant of the Board

Whenever, under this Act or any rule or regulation made or scheme sanctioned thereunder, the doing or the omission to do anything or the validity of anything depends upon the approval, sanction, consent, concurrence, declaration, opinion, or satisfaction of --- (a) the Board or the Chairman, or (b) any officer or servant of the Board, a written document signed in case (a) by the Chairman, and in case (b) by the said officer or servant, conveying or setting forth such approval, sanction, consent, concurrence, declaration, opinion or satisfaction, shall be sufficient evidence of such approval, sanction, consent, concurrence, declaration, opinion or satisfaction.