During the 1930s and 1940s many auto and truck manufacturers protected the long ignition wire between the dash switch to the coil beside the engine. The technology of wire insulation wrap during these early years was a woven cotton covering and it was more susceptible to damage from oil and antifreeze in the engine compartment. For protection, a metal wrap was placed around this electrical wire. See attached photos from a 1941-46 GM truck. A metal cap even covers the connection on the top of the coil for protection. It unsnaps to disconnect the ignition wire from the coil.
The cable system was very good 60 years ago, however, today it can create much frustration. The cotton insulation on the inside wire has deteriorated and pieces can drop away. In time the hidden wire gets exposed and may touch the outer metal cable causing a dead short. The engine stops with no notice. It may occur only on a rough road or during a fast turn. The engine may run well at idle or not run at all. One can imagine how difficult locating the problem can be. Opening the two ends of the cable and replacing the original wire may be good insurance. A modern vinyl covered wire will never be seen inside the original metal wrap.