It's a lead hem weight, also used for drapery. Essentially keeps fabric hanging in the desired direction.
Oops, simultaneous typing there. Anyway, they can sometimes be dated by numbers that are molded into them, but earlier ones were blank from what I've read. They lost popularity after the victorian era, when the Flappers came along (named so because they didn't mind if their dresses flapped around... Ok, I made that part up)
They are still used today, looking essentially the same, just a lot less prevalent.