Sorry, but to get a good ID we will need a clear picture of the front. Something for scale will help as well.
If the back is rusted, a little electrolysis will help.
If there is rust/crud on the front you can clean it with "The Must for Rust" from Krud Kutter. You can get it at most big box building supply stores. Spray on and use a nylon brush or tooth pick to get the stuff unstuck. Works on guilded buttons but will remove the patina from copper and bronze. Sometimes I just dip a toothpick in some and work on the spots. Just be gentle. Go slow. etc. It will work on the back as well but you may have to let just the back sit in a little puddle of it on a saucer.
Daryl