Another interesting thing about WW2 mail - they would photograph letters onto microfilm, then reprint them on the other end to save valuable shipping capacity.
> Portable readers are plastic devices that fold for carrying; when open they project an image from microfiche on to a reflective screen. For example, with M. de Saint Rat, Atherton Seidell developed a simple, inexpensive ($2.00 in 1950), monocular microfilm viewing device, known as the "Seidell viewer", that was sold during the 1940s and 1950s.
Apparently that’s not what they really used for mail in WW2, though. This video shows how it was really done.