Parker Pen Company uses casein.
In searching vintage Parker advertising, it was discovered that colorful cap tops were used about 1906 on Collegiate eyedropper pens. At this time Parker pens were becoming very popular, however George Parker always had his eye on other markets and applications. Collegiate pens were standard black hard rubber eyedropper pens made to offer school fraternities and sororities with color matching cap tops. It is believed that these buttons were made from casein, and are the earliest reference to casein use by Parker. At this time Parker also developed the Emblem pens, which carried fraternal and “secret societies” emblems fastened to the cap clips.

Around 1921 Parker introduced “Ivorine” pens for the ladie’s market, and they “sold like wildfire”. This November 24, 1921 advertisement shown here tells more of a story than just about “Ivorine”. Notice that 1,200 pounds of this material is being shipped from France and evidently was difficult to secure. “Ivorine” pens and pencils today are highly collectible and fountain pens in pristine original colors are very difficult to find.

1921 advertisement for Ivorine

Parker Pastel Ladies Pencils (collection of Jimmie Cockburn)

Today Conway Stewart has a new process of producing casein pens that is quite unique and they do not have the inherent flaws as production from early in the century.

References:
David Wells Vintage Pens
Jonathan Donahaye Conway Stewart Collection
Conway Stewart history
Home of the Plastics Historical Society
Plastics Historical Society – History of casein
Sandretto Plastics Museum Turin, Italy
Sandretto Plastics Museum casein artifacts
Jimmie Cockburn Pens and Pencils