No takers?
Well when I first saw it I couldn't think what was happening until I discovered that the Crystal Palace was home to the Imperial War Museum from WW1 until 1924 when it moved to Bedlam. The mystery photo predates the BTL occupation at the Palace and shows one of the more extreme tools for observing the enemy's position. It is a very long periscope. Also in the Museum's collection is a more down to earth tool for the same purpose,
the camouflage tree.
Peter