There is a brisk business on faked WW2 and post war material appearing in the last decade with overprints of concentration camps, swastikas, afrika corps and such on cheap Hitler head postage. Michel StampCatalogue, German Stamp Society and the Third Reich Study group are orgs I've found helpful. This overprint is a new one to me, so I'd seek around.
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Provisional stamps temporary approved after WWII until new stamps arrived