All bonds shall be signed by the chairman of the authority, and the official seal of the authority shall be affixed thereto and attested by the secretary of the authority. Any coupons attached thereto shall bear the signature or facsimile signature of the chairman of the authority. In case any officer whose signature appears on any bonds or whose facsimile signature appears on any coupon shall cease to be an officer before delivery of such bonds, such signature shall nevertheless be valid and sufficient for all purposes the same as if he had remained in office until such delivery.