These are imitations with a precious metal surface layer on top of a base metal core. They are frequently detected when the surface layer cracks or flakes away, exposing the copper-alloy core beneath. Corrosion of the underlying core may appear as bubbles or lumps on the surface, and the surface may be totally destroyed, leaving only the core. Plated imitations are typically lighter than the coins they imitate.
What does contemporary forgery mean?
Contemporary copies are those that were manufactured at the same time as or shortly after the official coins that they duplicate. Some are convincing and were most likely intended to pass as official coins, while others are so crudely reproduced that they would not have deceived anyone.