They are usually made from shim stock or thin gauges of steel fabricated to expand or enhance the magnetic field to the strings. Leo Fender used a ferrous metal claw around the pickup cover on the Jaguar and Bass VI guitar. The metal claw was used to concentrate and retain the magnetic field within close proximity to the coil. It would help enhance the output, brightness and clarity in each pickup. There are several after market products that attach to the pickup but have been known to cause the pickup to feedback uncontrollably. Fender also used a ferrous bottom plate on the lead pickup on Telecasters that acted as a means of grounding the strings and bridge plate and also enhanced the magnetic field towards the strings.