Interesting place for a soup can.
I have seen a firepalce damper similar to this but the soup can had been cut off already. Here is a link to the photo taken of it. you can see the ring left on the damper from the soup can. In this image the fireplace damper is a gas fireplace and it has the clip on it to keep the damper open. http://www.chimneyballoonusa.com/blog/2008/04/7-chimney-flue.html
I believe the soup can is a weight that is intended to keep the swivel type damper closed so downdraft winds in the fireplace chimney do not blow it open on accident.