It now looks like I have had direct experience with casein. Looking over the Red Bear web site, I can no longer find any reference to "Tortis." It now says "Our picks are made from Galalith also referred to as Milk Stone or Avoriolina (Ivory imitation). This natural product is made from the hardening of casein, the protein of milk which is a 'polymerized animal protein'." However, they very much used to use the Tortis branding, e.g.:
Attachment 186692
I first corresponded with Red Bear discussing "Tortis" a dozen years ago. Continued using it in correspondence as recently as late May and wasn't corrected, although I now note that the two folks from Red Bear with whom I was corresponding never mentioned "Tortis" in reply. The plot thickens . . . or thins. Whatever.