5923010 ?Artillery observation officer in forward post, our barrage during the advance?, 1917 circa (b/w photo) by Unknown photographer, (20th century); National Army Museum, London; (add.info.: ?Artillery observation officer in forward post, our barrage during the advance?, 1917 circa. Stereoscopic photograph, World War One, Western Front, 1917 circa. On the Western Front, British artillery fire was often directed from ?advanced observation posts? (OPs) situated ahead of the gun positions, enabling the battery to engage the target by indirect fire. Sometimes a post would be hidden out in no-man?s-land. From a collection of 101 stereoscopic photographs entitled ?The Great War? including the official series.); by National Army Museum ; out of copyright.