This is an 1880 MacKinnon “Fluid Pencil” eyedropper-fill stylographic pen in Black Chased Hard Rubber with (2) gold-filled repousse bands and repousse cap. Douglas MacKinnon is recognized as the inventor of the stylographic fountain pen (more here, thanks to George Kavalenko). The rear knob is turned slightly to allow air into the barrel and avoid vapor lock when the pen is in use. 1879 Patent date on gripping section. May never have held ink. Near mint+
