Demonstrator pen

Demonstrator pens are models which either are transparent or have cutaways making their internals visible.

These would typically allow direct viewing of the filling mechanism and the section-inner cap junction, respectively.

[1] The first transparent demonstrator pens were probably Parker's Bakelite-barreled eyedropper-fillers, which were soon made a regular production model.

Most transparent demonstrators, however, postdate the general adoption of celluloid for fountain pen manufacture.

Parker and Sheaffer both made fully transparent versions of their best-selling Duofold and Balance models in the early 1930s.