Demonstrators were originally not for sale. They were furnished by the manufacturers, and used as sales tools to show dealers, salesmen, and customers how a pen or pencil was constructed and how it functioned, and to instruct repairmen (David Nishimura). 

It seems that the earliest demonstrator were cutaway demonstrators, dating back to 1900's at latest. They had openings in barrel, allowing direct view of the filling mechanism. Many were furnished with nonfunctional dummy nibs or without nibs. That's how they were constructed, cutaway demonstrators were not fully functional.

AFAIK, the first transparent demonstrator was Parker Bakelite pen. In 1914, Parker introduced the Lucky Curve pens with a transparent Bakelite barrel. “Transparent barrel lets you see just how much ink there is in the barrel at all times and shows how the Lucky Curve drains back the ink into the barrel by capillary attraction. This Bakelite Pen was originally made for the use of our salesmen in demonstrating.  Later they were supplied to the dealers for the same purpose. Now we are manufacturing four numbers in Bakelite pens for sale to people who wish something distinctive and unique in the fountain pens they carry."(Parkersheaffer.com). As state above, Parker demonstrator were fully functional and later sold to connoisseurs. Later, transparent celluloid was used for barrel.

I'm afraid I digress. The early demonstrators for Pelikan pens were cutaway demonstrators. Pelikan 100 and 100N cutaway demonstrators are known⁽¹⁾. They had openings in cap top, cap tube, section, barrel, and turning knob. These allowed direct views of the filling mechanism and the junctions between each parts. Dummy pens were also made for a display.

In 1950's, Günter Wagner began to make transparent demonstrators for 100N and 400 using transparent acrylics. They were fully functional, but still not for sale.

Note;⁽¹⁾I suppose that demonstrators for Rappen was not made as its filling system was visible as it was. Please let me know if demonstrators for IBIS and AUCH-Pelikan were ever made.