WHAT IS THIS ?

I found this in an old New England farmhouse. It appears to be in very good shape for its age. The only markings are ,S + INT, IN , S with a wire that goes to GRD. The part that unscrews probably held a wire of higher voltage. When I unscrew the back it is solid like an old condenser. I hate to throw away old things like this. I would really like to know what it is used for. There must be some old timers out there who have seen one of these. thanks for any info.