During World War II, German military censorship was applied to all mail in occupied territories. Censors scrutinised entire messages and redacted prohibited content. To bypass these measures, individuals sometimes used invisible ink—also known as sympathetic or covert ink. However, this was a risky endeavour, as the Auslandsbriefprüfstellen (ABP) subjected approximately 3% of all mail to chemical testing to detect hidden messages.
Registered cover, franked with 2.50 Lit., sent from Laibach (Ljubljana) on August 28, 1944, to Franz, Steiermark (Vransko, Styria), arriving September 20, 1944. Processed by the Foreign Mail Examination Office in Vienna. The front features several censorship markings, including a dark blue 'Y' (laboratory) and red 'W' (supervisor review), along with a green examiner's stamp '8635'. The reverse shows a chemical censorship swipe (test for invisible ink). The cover was resealed with censorship tape, marked with a red machine-applied censor mark.
Vienna censor codes:
1000 - 3000 Regular Mail
4000 - 6000 Registered and Express mail
8000+ Specialised departments (Chemical/Language specialists)
