ferrite core (toroid) used as a base to wind an inductor, often for filtering high-frequency noise in power supplies, industrial gear, or automotive electronics, offering features like high current handling and low leakage. These are customizable components, available from manufacturers who specialize in coils and magnetic cores, found on sites like Mycoiltech or through custom design services. 

In short, you're looking for a specific size of toroidal core component for building custom inductors for electronic filtering.