Cadwell’s viewers were sold with sets of views (Kilburn views are specifically recorded with some Cadwell viewers), and extant examples turn up in museum catalogues and auction / collector listings.
Some surviving relics show a later-modified knob opening, added so cards could be flipped manually if needed. A practical detail that collectors point out.
The Smithsonian’s Patent-Model indexes list Cadwell’s patent among the Patent Model entries for 1874.
Images shown with permission from
@keitawangari at Vintage 3D Viewers. Patent schematic from the National Stereoscopic Association’s StereoWorld newsletter November-December 1977.