Series Number Id: UNKNOWN. Sheet number: part of sheet 36. First Army secret defence map. "[First] Field Survey Co., R.E. 5740 [numbers crossed out and replaced with blue overprint] 5839." Grey squared base map. German front line in red; British front and rear defence lines in red and blue; zones of defence in pink, pale blue, and pale green. Includes "Reference" to defended localities [hatched areas], assembly points, posts and wire., Location: Research Collections; Fonds: WW1 Trench Maps: France; Box number: PC23; envelope_number: 701.