Although these are labeled Suntour, they are also labeled as Pederson Self Energizing System brake calipers. Pederson sold his design to Suntour after introducing them to the market. You don't see them around much but they work well on a tandem. As the pads engage the rim, the caliper moves forward on a screw to increase the pressure. Originally there were front brakes too but Suntour didn't want to deal with the liability of the brakes not releasing or locking the front wheel. They are a challenge to set up but once they are set up right, they do perform well.

P1010340, on Flickr