Plated brass pickup covers to fit Telecaster neck pickups.
A high amount of eddy currents form in the brass as electricity forms inside your pickup. These eddy currents put a reverse force on the output of your pickup, leeching away some of the treble response. This is what gives Tele neck pickups their characteristic mellowness.
Brass is one of two materials regularly found in Tele neck covers - the other being nickel silver. Eddy currents also form in nickel silver, but to a lesser extent, making the same pickup comparably brighter. You can find Nickel silver Tele covers here.