Any notice, determination, direction, requisition, appointment, expression of opinion, approval or sanction, to be given or signified on the part of the Board, for any of the purposes of, or in relation to, any powers or functions with which it may be invested by notification under sub-section (2) of section 2A, shall be sufficient and binding if in writing signed by the Secretary to the Board, or by any other person authorised by the said Board to act in its behalf in respect of the matters to which such authorisation may relate; and the Board shall not in any case be bound in respect of any of the matters aforesaid unless by some writing signed in manner aforesaid.]