Slightly used cable actuated hydraulic disc brake calipers, post mount. They are sold under several names, Acor, Juin Tech, and Yokozuna. They weigh only about 145 g and don't require as much adjustment as regular cable disc calipers as the pads wear. I believe the setup is a little easier too. I had no issues installing and using them, just went to a full hydraulic setup. They were used less than 1000 miles. I did have to "customize" one of them by removing the plastic thumb adjustment knob, due to the design of my frame.