This Eastlake knob is different in that it has a smooth shanked bolt rather than a threaded one. The fastener is what is called a “finger” because it slips onto the shank, tightens up against the back of the door, and secured with a set screw. The knob was possibly used as a latch or catch, using the finger as a stop for a door when turning the knob.
The knob measures 1 1/8 inches across and projects 1 1/8 inches from the surface of the door. It has the brass cap, as most Eastlake knobs.
This is a very fine knob, well over 100 years old. The original black paint is all there!