Subject to the provisions of any other written law, any contract made in the Colony on behalf of the Government shall, if reduced to writing, be made in the name of the Government of the Colony and Protectorate of Kenya, and shall be signed either by the accounting officer or by the receiver of revenue of the Ministry or for the department of the Government concerned, or by any public officer duly authorised in writing by such accounting officer or receiver of revenue, either specially in any particular case or generally for any contracts below a specified value in his department or otherwise as may be specified in such authorisation.