I believe that is a hem weight.
Clean the dirt out of the hole in the middle. If it is a bar, and it's made of lead (white patina), then it is a hen weight.
The trade weights that I have seen, were not lead, and had marks stamped on them, to assure their authenticity. Sometimes hem weights have a letter or number stamped into them. The marks on a trade weight are daggers, lions, crowns and stuff like that... Royal symbols.