The cover is basically a component that protects the coil and especially helps keep the string from getting snagged under the lip of the pickup. The Duosonic and Mustang have basic single coil pickups using .625″ length rod magnets. The pole pieces are flush with the top of the pickup so there is no need to have a cover with holes. The solid top cover is important for Mustang guitars because the switching circuit could reverse the electrical phasing of the coils. When reversing the lead wires, the pole pieces will buzz if touched.

The solid cover eliminates accidental touching of the pole pieces. Also the solid covers help keep moisture from penetrating down into the coil which could eventually cause ICPC within the inner coil. ICPC is “Inner Coil Pole Corrosion” which I have found can cause perfect looking pickups to fail. ICPC is caused when moisture rusts the magnet at the coil beginning and breaks down the insulation on the magnet wire eventually causing the coil to fail.