I think that is a cable operated rear disc brake; back in the 80's I got a pair of them and brazed their mounts onto a Sekine and built a disc-braked road bike. I used a rear hub on the front as it had threads for the disc, had to braze one cone onto the axle so it would be narrow enough to fit the forks; and I turned threads on the other side of a freewheel hub for the rear disc. A bit heavy, but nobody else had such a bike - and the braking was actually better, in that there were no rim-induced variations in the brake friction.