Signal Corps IN-63. Used at least as early as 1919 well into WWII, for field communications. The composition is made of asbestos, mica and shellac. More information can be found in Gerald brown's book on non glass, non porcelain insulators. This insulator looks like it led a sheltered life. Appears to be in "never used condition". I'll ship this little gem to you for $25, U.S. or Canada