Elastic Capos with elastic straps Capo ad from the late 1890's. Transposition Roller US patent in 1897 by Lincoln Utt. This indicates that curved fretboards were in use at the time.
Tension is created by a coil spring.
Between the back of the neck and the spring is a thin metal plate, either for neck protection or easier sliding. The spring is held in place by outward bent ears on the plate. You can see the slotted ends of the plate on each side of the roll . It seems to be a real laquer scraper, but maybe the neck was protected by a pad. The coil spring is stretched by a lever, just like an old ski fastening.