Some pre-1900 furniture knobs have a special nut with a set screw. This finger is on a smooth shank (not a threaded screw). This piece tightens the knob into place and may also be used as a catch to keep the door shut.
This 1 1/4 inch knob is Eastlake, dating from 1870 to 1890. It has a brass cap with the typical Eastlake pattern of star points. The wood knob was originally painted black and retains all of its paint.
The knob is in very good condition.