17.(1) Postage stamps provided under section 16 shall be deemed to be stamps issued by Government for the purpose of revenue within the meaning of the Penal Code, and, subject to the other provisions of this Act, shall be used for the prepayment of postage or other sums chargeable under this Act in respect of postal articles, accept where the Government directs that prepayment shall be made in some other way. (2) Where the Government has directed that prepayment of postage or other sums chargeable under this Act in respect of postal articles may be made by prepaying the value denoted by the impressions of stamping machines issued under its authority, the impression of any such machine shall likewise be deemed to be a stamp issued by Government for the purpose of revenue, within the meaning of the Penal Code.