Demonstrator pens were usually clear pen models used by sales clerks to show customers how the insides of a pen works, to show off the parts of a pen, how a particular filling mechanism worked, and more. Check out our post on Waterman’s Ink-Vue demonstrator pens.
Some demonstrators weren’t clear, but were actually full pens with parts cut away so the inside could be seen. It wasn’t that long ago that this practice was stopped. In the late 1950s, manufacturers still did this.