Watten Bridge Head - Merckeghem Switch - St. Omer - Sercus Line
Series Number Id: UNKNOWN. Sheet number: parts of 27 SW, 27 SE, 36a NW, 36a NE. Secret, Second Army rear defences, July 1918. "Secret" in red overprint, along with title statement. "[4th] Field Survey Bn. R.E. (9456C) 11-7-18.", Location: Research Collections; Fonds: WW1 Trench Maps: France; Box number: PC21; envelope_number: 674.