Hi; I may be wrong here but I doubt it is a button. There is no evidence of ever having had a backing or shank ever attached or any evidence that a backing plate was ever attached as well. The rim would show a lined type of patina or some type of concentric edging marks around it's edges. They are not there. I believe it is a child's toy metal play plate. It would have been part of a set. I think it is 1920's or so. Neat find. PEACE:RONB