JF Ptak Science Books Quick Post
“Public Warning” was a large, billboard-sized poster that appeared throughout England, posted in the first year of the war, appeared in Technical World Magazine in May 1915. Outside of delivering some good, solid information on behavior and bombing it supplied German/British aircraft silhouettes to help people distinguish between friendly/enemy aircraft, meaning that they could take cover and report the actions of enemy aircraft (valuable information in pre-RADAR days) and also not fret with British aircraft sightings.
The National Archives (U.K., http://www.nationalarchives.gov.uk/pathways/firstworldwar/transcripts/spotlights/public_warning.htm) transcribes the poster, identifying the aircraft (just in case you can’t read the text on the poster):