The second photo shows a photo of a J-D Petticoat type spark plug. It's interesting to note that Champion acquired this company in 1916 so they could obtain the rights to mineral deposits of Siliminite, which produced a superior insulating material. The "Petticoat" type of plug has a deep recess surrounding the center electrode. That provides a greater surface area a spark would need to travel in order to short out. Spark will travel over the shortest distance, and in the case of a "Petticoat" style plug, that distance will be between the center and ground electrode-just where the spark should be.