27. Meaning of service by post.- Where any (Central Act) or Regulation made after the commencement of this Act authorizes of requires any document to be served by post, where the expression "serve" or either of the expressions "give" or "send" or any other expression in 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.