What type of site did you find it at ? And what other finds have you found at that site ? From looking at the pictures it appears to have had something else attached to it at some point on the inside ring it looks like half a hing attached to it .. I would try a little more carefull cleaning maybe with a tooth pick and gently kind of brush away a little of the rust and see if there is a pattern on the iron part of it .
Seems like there was something attached to the top of it, because of the large iron blob. Maybe it opened and there was a mirror inside. I found artifacts from 1600,s period.