If genuine, it is indeed a German WWII propaganda forgery.
The exact date of their manufacture isn't known. The underlying stamps are
German forgeries of the British 1937 King George VI issues. They can easily be distinguish from the real issues by the inclusion of a Star of David atop the crown at top center and a hammer and sickle inserted in the flower at top left and in place of the pence symbol. These forgeries are believed to have been printed by prisoners at the Sachsenhausen Concentration Camp.
The Germans then overprinted a small quantity of the forged stamps with propaganda slogans. To date, 32 overprint types are known, grouped in 5 series. With 32 overprints applied to 6 values of stamp, there are 192 possible combinations. Michel catalogs them by denomination (MiNr 9-14), then Series (I-V), then overprint type (a-f). The one shown at the link is MiNr 11 IV c.

The 5 series are (estimated quantities in existence are in parentheses):
Series I -- Bombs
a. Cathedral of Rouen (30)
b. Castelle Candolfo (?)
c. Monte Cassino (30)
d. Schaffhausen (30)
e. San Marino (30)
f. Cathedral of Cologne (30)
Series II -- Extremes of World Politics
a. World-Bolshevism (30)
b. World-Capitalism (30)
c. World-Judaism (30)
d. 14.8.1941 The Bluff-Charta (30)
e. Empire's Liquidation at Teheran (30)
f. World Slavery (30)
Series III -- Invasion (a.k.a.
AAAO Overprint)
a. But who will return? (6?)
b. On order of Stalin (6?)
c. England bleeds on the order of Moscow (6?)
d. Come-on Germany expects you (6?)
e. England has lost the war (6?)
f. A military adventure (1)
Series IV -- Liquidation of Empire, Series A
a. Singapore (400)
b. St. Lucia (400)
c. Barbados (400)
d. Jamaica (400)
e. Trinidad (400)
f. Bahama-Is. (400)
g. Bermuda-Is. (400)
h. Hongkong (400)
Series V -- Liquidation of Empire, Series B
a. Rabaul (400)
b. Borneo (400)
c. Grenada (400)
d. Rangoon (400)
e. Bougainville (400)
f. St. Vincent (400)
Of the series, the Liquidation of Empire are, by far, the most common. They catalog for €85/€100 each, and they can commonly be found at auction in strips containing multiple overprint types of a single denomination.

The Bombs and Extremes of World Politics are less common, cataloging MNH for €300 (Bombs) and €280 (Extremes, except Judaism at €380).
The Invasion series is the rarest, at €1,200 MH, except "A military adventure" at €3,000.
See also
Propaganda and Espionage Philately By SGM Herbert A. Friedman (Ret.)