136. Whenever any sentence of transportation or rigorous imprisonment is passed under this Act, or whenever any sentence of death so passed is commuted to transportation or to rigorous imprisonment, the commanding officer of the person under sentence, or such other officer as may be prescribed, shall forward a warrant in the prescribed form to the officer-in-charge of the civil prison in which such person is to be confined, and shall forward him to such prison with the warrant: Provided that in the case of a sentence of rigorous imprisonment for a period not exceeding three months, the confirming officer, or in the case of a sentence which does not require confirmation, the court, may direct that the sentence shall be carried out in military custody: Provided further that, on active service, a sentence of rigorous imprisonment may be carried out by confinement in such place as the officer commanding the forces in the field may, from time to time, appoint.