In searching the ICON pages for information on this piece I found the following posted by Doug MacGillvary:

"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."

Appearently, these came with a porcelain sleeve that has a threaded nut for maybe clamping a wire in the upper notches? Looks to be brand new. $75 plus shipping for this rare contraption. SOLD - thanks for looking!