13Every document purporting—
(a)to be an instrument made or issued by or on behalf of the Authority and to be sealed with the seal of the Authority authenticated in the manner provided by paragraph 11 above, or to be signed or executed by the secretary or any person authorised by the Authority to act in that behalf, or
(b)to be such a certificate as is mentioned in paragraph 12 above,
shall be received in evidence and be deemed without further proof to be so made or issued or to be such a certifcate, unless the contrary is shown.