1931, philatelic exchange cover from Moscow (originated from Kharkov) to Germany, bearing on reverse black surcharge 5k on 100r yellow orange, imperf at top left corner, tied by 8-line official cachet, franked by six values, including three stamps of the Hague Air Conference 10k blue and light brown, all appropriate markings and arrival ds, mostly VF, Standard Cat. #PE14, Est. $140-$180 (Image)