Great Britain Shipwreck Mail, with "by Bottle" label

This British Back of the book label was issued in 1944 during World War II when shipping losses to German U-Boats was actually decreasing. Nevertheless the Crown Agents issued special stamps to cover the eventuality that shipwreck could occur to maritime traffic heading across the Atlantic from the colonies. These stamps or rather labels were printed on Parkinized paper and with waterproof inks. They were however extremely unpopular with sailors who regarded it as tempting fate.
 No known used copy survives today. Yet to be found any used on cover.
The short-lived issue, had a re-invention when the design was slightly modified for the extremely popular Victory Omnibus issues of 1946.