Paper envelope, containing clear cassettes for use with photo-reactive film as ground zero nuclear blast indicators. Used by Royal Observer Corps and UK Warning and Monitoring Organisation.
Rectangular, manilla paper envelope, with opening and flap at one end. Each envelope contains four, clear perspex / plastic rectangular cassettes with printed horizontal and vertical calibration lines delineating compass bearing and elevation above the horizon. Each cassette marked with either 'N', 'E', 'S' or 'W', indicating direction. Each cassette wrapped in brown, waxed paper.