The circular steel plate carries the hand-stamped serial number, but also holds the fitting that clips to a saxophone strap to hang the guitar from one point at its center of gravity.
Yes, a sax strap. So, you ask, how do you keep the guitar from sliding around while you are picking and fingering? Why, you cover the back with gray mastic and glue some rough horse hair (upholstery padding) in place.
Just don't play it bare-chested, that's all. Ouch!