27 Meaning of service by post. Where any Central Act or Regulation made after the commencement of this Act authorizes or requires any document to be served by post, whether the expression serve or either of the expressions give or send or any other expression is used, then, unless a different intention appears, the service shall be deemed to be effected by properly addressing, pre-paying and posting by registered post, a letter containing the document, and, unless the contrary is proved, to have been effected at the time at which the letter would be delivered in the ordinary course of post.