Located in Maine. Pitch is A450. A small metal arm with a knob protruding from the treble key cheek operates a sliding metal plate with holes in it, over the treble half of the primary swell shade, which has corresponding holes. =differential enhancement of treble. [Carhart patent 17,196 - May 5, 1857]. Secondary horizontal swell bar opens at top of the throw of the long arm operating the primary swell shade, via a vertical rod which impinges on the short arm of the L.