Campagnolo Serrated Brake Mounting Washer
A heavily toothed mounting washer. It's a Campy one, but works with any caliper brake. If you ride in the winter and your brake hardware has disintegrated from corrosion, try these ones. They also work really well to stop the caliper from squirming around during adjustment. Machined. Made in Italy.
Campy's part number is 25-BR-RE021. Schematic is shown in the second picture.