Series Number Id: UNKNOWN. Sheet number: part of sheet 36. Secret, Second Army railways, June 1918. "[4th] Field Survey Coy., R.E., (8648) 12-4-18 [followed with red overprint] (8991) 12-6-18". "Ordnance Survey, (O.B.) April, 1918." Grey base map with olive-green squaring; overprint in purple, red, and green., Location: Research Collections; Fonds: WW1 Trench Maps: France; Box number: PC22; envelope_number: 699.