(a)No communication of the nature of a letter or having the character of a personal communication shall be enclosed in or written upon a book packet except to the extent specified in rule 17.(b)No card bearing the heading "Post card" shall be transmitted open as a book packet.(c)A book packet shall not contain "paper money" which for the purposes of this rule includes unobliterated postage or other stamps, currency notes, hundies, cheques, bank notes, bank post bills, bills of exchange, and all orders or authorities for the payment of money; provided that a book packet may contain one stamped and addressed envelope, post card or wrapper bearing the name and address of the sender of the packet, or some other person named thereon.
