Antique Stiffel Lamp

Three-pole adjustable Stiffel desk lamp, primarily brass.  Multi-position knob on base allows various combinations of the three bulbs for more or less light.  Knob atop shade allows shade height to be adjusted.  Each bulb socket has a "Stiffel" label affixed.

This lamp is fully functional and feels very solid, though it is fairly dirty as I'm not sure what (if anything) should be used on it.  The shade seems to be original, black exterior with fine gold trim and a light-colored interior.  Some gold trim on the shade is damaged and there is some staining on the inside of shade but it is overall intact.

Any additional information (and valuation, of course) would be appreciated.