This is a fantasy item based on real propaganda artwork used on leaflets dropped over Allied troops in Europe (to undermine their morale). Such a postcard in 2nd half of 1944 would have certainly be liked by Germans mercilessly bombed by UK and US air-force for years. Truly a "vengeance"  (Vergeltungswaffe) weapon (thus the name - V-1). 
V-1s started falling on London on June 13, 1944 - a week after the D-Day landing. Although formidable, due to the scale did not change the course of war, not English determination to win the war. At peak, more than one hundred V-1s a day were fired at southeast England, 9,521 in total, decreasing in number as sites were overrun until October 1944, when the last V-1 site in range of Britain was overrun by Allied forces,