This is a project by Brad Burt describing how he cloned a Rickenbacker bass neck pickup. The project was initially going to be a simple rewind of a Rickenbacker pickup bought on e-bay, but Brad decided to make a new one and at the same time change the design slightly to improve it. For example Brad used copper covered circuit board for the top flat-work - which would provide some sheilding from noise. The pole pieces are made from low carbon steel, which Brad argues is a more appropriate alloy than Rickenbacker used on their rivet pole pieces

Link: Brad Burt shows how to make a Rickenbacker bass pickup

On their single coil high gain (button top) guitar pickups Rickenbacker used 44 AWG formvar wire, wound around individual steel pole pieces to a resistance of around 7K to 10K Ohms. The magnets were ceramic. Presumably the bass pickup uses some similar materials.

Brad Burt runs Classic Amplification, a small custom-shop style business located in Sammamish Washington USA, doing custom builds, repairs, and mod's of Tube Amps, Effects, Pickups and Guitar work. Link: Classic Amplifaction Website