1943: the Bank of England never found them. Plans had been made to drop British pounds over London by plane. Today, these notes are considered very valuable by collectors. An industrial area, outside the western camp perimeter, contained SS workshops in which prisoners were forced to work; those unable to work had to stand at attention for the duration of the working day. Heinkel, the aircraft manufacturer, was a major user of Sachsenhausen labour, using between 6,000 and 8,000 prisoners on their He 177 bomber. Although official German reports claimed the prisoners were "working without fault", some of these aircraft crashed unexpectedly around Stalingrad and it is suspected that prisoners had sabotaged them. Other firms included AEG and Siemens. Prisoners also worked in a brick factory. Prisoner abuses Overall, at least 30,000 inmates died in Sachsenhausen from causes such as exhaustion, disease, malnutrition and pneumonia, as a result of the poor living conditions. Many were executed or died as the result of brutal medical experimentation. In 1937, the SS constructed a Cell Block for the punishment, interrogation, and torture of prisoners. Important people confined there including Martin Niemöller and Georg Elser. From 1939 until 1943, over 600 homosexual prisoners were killed. In November 1940, the SS executed 33 Polish prisoners by firing squad. In April 1941, over 550 prisoners were killed under Action 14f13. In the autumn of 1941, over 10,000 Soviet prisoners of war were shot. In May 1942, the first hangings commenced from gallows in the roll-call area. These continued until 1945. In May 1942, 71 Dutch resistance fighters and 250 Jewish hostages were executed. In May 1942, "Station Z" was completed in an industrial yard outside the camp walls. It included prisoner killing rooms, four crematoria, and a gas chamber after 1943. In 1941, an adjacent sand pit was enlarged and made into an "execution trench". Camp punishments could be harsh. Some would be required to assume the "Sachsenhausen salute" where a prisoner would squat with his arms outstretched in front. There was a marching strip around the perimeter of the roll call ground, where prisoners had to march over a variety of surfaces, to test military footwear; between were covered each day. Prisoners assigned to the camp prison would be kept in isolation on poor rations and some would be suspended from posts by their wrists tied behind their backs (strappado). In cases such as attempted