It is attached to the telephone line. There was insulation crammed up in the floor joists, and it made it look like there are a K&T circuit running to it. It turns out the K&T circuit does a 90 degree change in direction when it gets up in that set of floor joists.
I did check and the phone line is going through this lightening arrestor and still being used.
Wow - learn something every day.
I took another pic with my cell phone and it did come out a little better, but I forgot to email it from my phone.