From Rocklopedia Fakebandica
String-Horn, from "The Horn of Plenty of Trouble" story of Police Comics #87 (February 1949). It's a bizarre combination brass and string instrument much desired by criminal Phonie Le Ghoum. Le Ghoum even kidnaps Casimir's girlfriend Joujou to get it. Plastic Man rescues her, and it turns out Le Ghoum wants the String-Horn, because when played, it makes locks unlock.
Casimir's last name is never given.