Described as “a forgotten masterpiece” German Concentration Camps Factual Survey was commissioned by the British army film unit as documentary evidence of Nazi crimes – to prove, beyond doubt, that they actually happened. It was made in the immediate wake of the Holocaust and the liberation of the camps by the allies in 1945. Directed by Sidney Bernstein of the British Ministry of Information, the documentary also had a very famous producer – Alfred Hitchcock – who advised, for instance, on how to avoid accusations of camera trickery. But Bernstein’s film was never finished and it isn’t entirely clear why (although there is evidence of a Foreign Office memo advising against it). Some of the film was used in Billy Wilder’s Death Mills and…