It isn't horse tack related...at least it's nothing I am familiar with. As I posted it reply to your "What Is It" thread, I think it is a contact from inside an old flashlight that would have taken D cell batteries. It has a definite rim around the perimeter, and the nipple looks to be a bit concave, which would have been caused by battery contact fatigue. The rim is probably a spring seat where the coil spring was seated.