Asbestos was commonly used as an insulation and fireproofing material during the war; most Royal Navy vessels included a significant amount of asbestos as insulation for pipework and to fireproof magazines (as an aside, the Royal Canadian Navy included a ship literally named Asbestos, after the mining town of Asbestos, Quebec). However, this joke is not really related to the war. Rather, it refers to the fireproof curtains used in theatres. These curtains are supposed to prevent fires on the stage spreading into the auditorium. Such curtains were typically made of asbestos before the cancer risks were well-known.