Save as expressly provided by any rules or regulations under this Act, any order or determination required or authorised to be made under this Act by any air-force, naval or military officer or authority may be signified under the hand of any officer authorised in that behalf; and any instrument signifying such an order or determination and purporting to be signed by an officer stated therein to be so authorised shall unless the contrary is proved be deemed to be signed by an officer so authorised.