During WWII, all mail was censored. Censors read entire messages and censored prohibited content. One way of secret communication was to use invisible ink (also known as covert ink, sympathetic ink and white ink). However, this was dangerous because about 3% of all checks held a chemical test.
Registered letter, franked with 2.50 Lit, sent from Laibach =Ljubljana (28.VIII.44) to Franz Steierm =Vransko Štajerska (20.9.44), censored by OKW Vienna*. Chemical censoring was applied on the back.
*OKW codes and offices
a - Koenigsberg --- Baltic States, USSR
b - Berlin --- Airmail transit – N. & S. America
c - Cologne --- Holland, Belgium, Luxembourg, N. France
d - Munich --- Italy, Spain, Portugal, Switzerland, Vatican
e - Frankfurt --- S. France, Switzerland, N. & S. America
f - Hamburg --- Scandinavia
g - Vienna --- Balkans, Hungary, Turkey, Bulgaria, Greece
h - Berlin --- (special for P.O.W. mail in late 1944)
k - Copenhagen --- Sweden, Norway, Finland
l - Lyon --- S. France
n - Nancy --- S. France, Spain, Portugal
o - Oslo --- Sweden, Finland, Denmark
t - Trondheim --- Norway to Sweden and Finland
x - Paris --- Red Cross mail, France
y - Bordeaux --- SW. France
