A = Sheets with alpha-numeric squaring system ; B = Sheets with National Yard Grid at 5000 yard intervals ; C = Sheets with National Grid. Many of the maps seem to have been mounted on linen by Guy Messenger - this has been ignored in the cataloguing. Most of the maps were stored in large slip-cases made by Guy Messenger made out of cardboard (sometimes recycled corn-flake cereal boxes) and tape or sticky-back plastic (usually black), but only samples of these boxes have been retained. Maps at Maps.KGM.18.C were listed by Map Department staff in May 2019 and sheet titles are taken from the cover (the original cover or a Guy messenger cover unless absent in which case they are taken from the map). The absence of an original cover has not always been noted.