165. Whenever any sentence of imprisonment is passed under this Act, or whenever any sentence so passed is commuted to imprisonment, the confirming officer or such officer as may be prescribed in this behalf, may direct either that the sentence shall be carried out by confinement in a civil prison or by confinement in a Military or Air Force prison, and 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 prison in which the person under sentence is to be confined, and shall forward him to such prison with the warrant: Provided that in the case of a sentence of imprisonment for a period not exceeding three months, in lieu of a direction that the sentence shall be carried out by confinement in a Civil, Military or Air Force prison, a direction may be made that the sentence shall be carried out by confinement in Air Force custody: Provided further that on active service a sentence of 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.