Not surprising; there were very few of them and they weren't in service long. Here is the Imperial War Museum's note about the example in its collection.
Early First World War period German protective steel helmet. German Army Group 'Gaede' (named after its commanding general) had suffered large losses in 1915, particularly by men suffering head wounds from blast-loosened rock fragments. Frustrated at having no effective replacement form of head protection, a Lieutenant-Colonel Hesse took charge and had 1,500 improvised helmets made at the Group's artillery plant at Malaise. The end product, the distinctive 'Gaede helmet' saw only very limited service (less than a year) before the M1916 Stahlhelm became a universal replacement to all German troops. The small numbers of Gaede helmets then in service were withdrawn and melted down, as a result few have survived. Recognised as one of the very first experiments in soldiers' head protection, the Gaede represents an important step in the history of combat helmet development.