Watten Bridge Head - Merckeghem Switch - St. Omer - Sercus Line
Series Number Id: UNKNOWN. Sheet number: parts of 27a SE, 27 SW, 36d NE, 36a NW. Secret, Second Army rear defences, July 1918. "Secret" in red overprint, along with title statement. "[4th] Field Survey Bn. R.E. (9457B) 11-7-18." Grey base; yellow contours; trenches shown in red; MG positions and lines of fire in blue., Location: Research Collections; Fonds: WW1 Trench Maps: France; Box number: PC21; envelope_number: 673.