Military post offices on the Salonica front. Cover fr. with 25c. and posted from "VOJNA POSTA*Br.999*19.9.17" with Serbian censorship "PREGLEDALA/VOJNA CENZURA (49)" and French "OUVERTpar lAUTORITE MILITAIRE*309" and tape, arr. "Geneve.*25.X.1917". German spies operating in Geneva during WWI occasionally managed to intercept Serbian correspondence to or via Switzerland. Letters and postcards acquired in that way were given to professionals in Serbian language. Their duty was to read mail carefully and to select letters contenting information that potentially could be of interest for intelligence service. After reading they labeled each item with short description.