The ORIGINAL pickup wire. This is what was used in the earliest days of pickup winding. Prevalent in all original pre-CBS Fender pickups, and used by many other brands throughout the 1950's and 1960's. If you want "vintage accurate", this is it. The coating varies slightly in thickness, and is generally thicker than the coating on other pickup wire. It makes it difficult to accurately reproduce results across multiple pickups, and can lead to a slightly brighter tone.
Each 1/2-pound roll has over 25,000 feet of wire, enough to wind 5-6 single-coil pickups, or 2-3 humbuckers.