GSGS (Series) ; 2742. Sheet 28SW. Ed. 5A. Trenches to 1st April 1917. OS April 1917. Belgium & part of France. German trenches red, with blue names; British front & support trenches blue. Situation just before the Battle of Messines, when the Ridge was blown up by 19 huge mines. MS on face: forward & rear microphone bases of 'W' Sound Ranging Section 1917-18. MS on reverse: C.