Where any enactment requires, expressly or by implication, that a summons in respect of an offence shall be issued or served within a specified period after the commission of the offence, and service of the summons may under the rules be effected by post, then, if under the rules service of the summons is not treated as proved, but it is shown that a letter containing the summons was posted at such time as to enable it to be delivered in the ordinary course of post within that period, a second summons may be issued on the same information; and the enactment shall have effect, in relation to that summons, as if the specified period were a period running from the return day of the original summons.