In March 1945, Allied forces finally crossed the Rhine River into Germany. After gaining a foothold, two large Allied armies swept into Germany -- the American 12th Army Group from the southwest and the British 21st Army Group from the northwest. As the armies sped through Germany, they bypassed and eventually surrounded Field Marshal Walter Model's German Army Group B, which was positioned in the Ruhr Valley.
This area of encirclement would come to be known as the Ruhr Pocket.
For three weeks in early April 1945, these 430,000 Germans were cutoff from the outside world. For reasons that have been lost to history, the Army Group B Postmaster determined that they were in need of provisional permission stamps for use on airmail originating in the Pocket (though regular Feldpost mail was free of charge, airmail service was not). So, he commissioned the Kämper printing house in Meinerzhagen to overprint the 1941 3 Pf Hitler Head (Michel 782) with "Feldpost" for use as an airmail stamp.
The provisional stamps were issued to troops in portions of the Ruhr Pocket, but the airmail service never materialized due to the war situation. So, they cannot be found validly used, though CTOs exist.
Two major variations exist -- one with smooth gum (Michel 17x) and one with vertically ribbed gum (Michel 17z). Additionally, a rare variation exists with inverted overprint (Michel 17zK).