To many, this is commonly called a crown race. We choose to call it a baseplate as ours is technically not a race. It is the foundation of the venerable Chris King Sealed Bearing headset and is designed as a low cost, sacrificial piece, easily replaced when worn or damaged upon removal. Occasionally, it’s nice to start with a fresh one - here’s where you’ll find them.

for more info. http://chrisking.com/parts/baseplates