Here showing are three demonstrator mechanical pencils, circa 1940-1955. Top piece is a Parker utility pencil with cutouts top and bottom to view the operation of the mechanism. The middle and bottom pieces (Sheaffer Snorkel and Parker 51) demonstrate mechanical operation by a view through their clear barrels. Pencils of this design were never sold in their day. They were shown by sales staffs to educate retailers and or their customers as to the internal workings of the devices. Modern demonstrators (clear barrels), produced since the last part of the 20th century are designed and sold as a style enhancement gobbled up by customers.