This is an awesome impurity piece; best one I have found in my 10 years of collecting. Two ICONners Mark Storaasli and Lee Southern recently identified the foreign body as a medicine bottle/vial corkscrew and Lee Southern more specifically as a Clough patent medicine corkscrew. Here is a picture of what the Clough corkscrew looks like without the glass encasement. [id=250515153].
Here is a weblink with great information on Clough corkscrews as well as other corkscrew information:
The insulator has a half a thumb nail size flake off the rear skirt, but who cares !! Also ghost embossed ROOK on the dome. Closeup of dome [id=221955819]
No longer available. Insulator has been consigned to an upcoming auction.