During the Second World War and some years after it there was a scarcity of paper. Different solutions to solve the problem can be found among mail. One of the solutions was – mainly during the war – to re-use labels from previously used envelopes. One find them very often with official mail, here in 1944 (1a/b).
An other solution was the usage of old military maps. In this case an allied map of Holzminden, Germany (between Hannover and Kassel) has been used in 1948 (2). Because it is a window envelope, it is quite attractive!
A very interesting invention was the Briverto. A stationery sheet with a window, so that the message could be typed and after folding it could be sent as if it was a letter in an envelope, but no envelope was needed! The example here (3+4) is from 1947, but according to a small add in Leidsch Dagblad of 14 April 1943 (5) it was probably invented during the War.