The brass plating is caused by zinc planchets left in the copper electroplating bath and dissolving over time. The weight is indistinguishable from a normal Zincoln and the only way to determine with 100% accuracy is an XRF test to analyze the plating composition. However, when you see an example that is really yellow and still has nice luster such as the one posted here, it is most likely a brass plated specimen.
























