I didn't know what this was when I picked it up at an antique bottle show. Answer: It is known as a "lightining rod stand off insulator" or is more commonly called a 'lightning rod insulator' and abbreviated as LRI. A mounting bracket fit over the top and captured the base. The bracket was nailed or otherwise attached to the side of a building or church. The conductor attached to the lightning rod was placed in the groove on top and the tie wire was looped through the hole to keep it in place. This particular one is known as a Tall Boy Slash Top. Many thanks to all who answered this question - particularly Glenn Drummond, Terry Drollinger, and Jim Colburn - the LRI experts.