Rod,
In short, that's a forgery of one of the Inselpost issues. As to what an Inselpost issue is...
The underlying stamp is a Zulassungsmarke, or permission stamp. During WWII, German troops could send mail up to a certain weight free through the Feldpost system (costs did accrue if overweight). But, for each package, they had to have a permission stamp allowing them to send the package (or multiple permission stamps if overweight). This allowed the Feldpost system to control the volume of mail by limiting the number of permission stamps issued.
Following the reconquest of Greece by the Allies, German forces stationed on Crete and the Aegean Islands could only be supplied by air. The army's field postmaster (Feldpostmeister) for these regions, Dr. E. Schwarz of Army Group F, decreed that only mail with permission stamps overprinted "Inselpost" should be transported. To accommodate this requirement, 200,000 permission stamps were overprinted in Vukovar, Croatia, for distribution to the troops.
Vukovar Inselpost Issue (MiNr 6)See also my website
here.
However, the aircraft transporting the stock of stamps was shot down during a supply flight in October 1944. The only copies of the Vukovar printing which survived were a small stock of approximately 2,000 stamps which remained in Vukovar. These were transported to Rhodes in December 1944, where they were distributed to collectors.
As a result of the loss of the Vukovar stocks, until a new supply of stamps could be distributed, several local issues were used. Among these were the November 1944 "INSELPOST" overprints which were used on the island of Rhodes. A total of 60,000 were printed, of which the majority were destroyed during the war.
Rhodes Inselpost Issue (MiNr 9)See also my website
here.
Finally, in November 1944, a new printing was done at the state printing office in Agram (Zagreb), Croatia. A total of 1.5 million were printed for use throughout Crete and the Aegean Islands.
Agram Inselpost Issue (MiNr 10 A b II)See also my website
here.
I should also mention that Inselpost issues aren't limited to overprints of the brown permission stamps, but those are the only ones I showed because that's what your forgery is. To see the remainder, you can go
here and get to them from the "Feldpost" menu.