Series Number Id: UNKNOWN. Sheet number: part of sheet 36. Enemy battery positions April 1918. "Field Survey Co., R.E. 6162 [number crossed out and replaced with blue overprint] 6176." "The portion 36a S.E.4 is from a recent survey, the co-ordinates of points in this area differ from those of the regular series sheets." Detail, grid and contours grey; overprint blue. [Enemy Battery] Positions Map, positions active, and positions inactive, between 9-4-18 & 27-4-18. Trenches to 27th April 1918. La Bassee Canal, railways and zone call areas in blue., Manuscript in blue crayon on reverse: O.C. 20 S.B. [Siege Battery]. WW1 Trench Map Collection: Box PC23, Envelope 716.