I found this unique insulator in a box along with the insulator in the previous picture. What is it?

That's probably a lightning arrestor with the top porcelain cap (adhered with tar originally) and another cap for its bottom (for the grounding part). Judging from the chocalate color, molding of the "ribs" and metal cap ends (I've seen these broken off), I'd say this was made by GE.

GE also made a "spark arrestor" whereby the body was identical to your piece. A brass cap was secured with tar to both ends with solid brass forks sticking upon each side at close distance (one was the high-voltage side, the other ground) which also acted as a lightning arrestor.

The indentations on your piece allowed for a secure surface between the tar sealant and mating surfaces.

Thanks to Joe Maurath