Metal 2.1mm DC switching jack, commonly used on effects pedals. The quality of these matches their look: sleek, elegant, and professional.
The metal bushing is independent of the power contacts. Some applications will require insulating the bushing from the chassis, so that the shell of the plug won't short to ground.
Tech's notes: "The switching portion allows you to wire to the battery. Then the circuit to the battery will open whenever something is plugged in, preserving battery life."