We have a vintage / antique cast iron ornate metal knob.  I'm not sure what it was on originally.  There was something screwed onto the top and the bottom (narrower end) attached to another piece......  It measures about 4" in L from the bottom to the top of the exposed screw x about 1 1/4" across the center top and about 1/2" across the bottom.  It has an ornate design through out.  It is a mix of the original finish and rust. 

Knob Ornate Cast Iron Vintage Salvage