This, I believe, is an electric fence tension spring. It is 9" long. The little spool insulators are marked BIRNBACH RADIO. I have learned that the Birnbach Radio Co. started business in 1923. Apparently they made an assorted line of electrical items. My question... Is this tension spring a product of Birnbach Radio or did they just supply the porcelain spools to another manufacturer. Also, can anyone narrow down the period of time this item was made. 1920s, 1930s??? One other thing...since this is the first time I've run across something like this, I wondered if it is pretty common? Any collector of wierd stuff have one of these?