if the clearcoat peeled off the top (happens in older units) then the top was lightened by the sun and other corrosives - the parts that were still fully clearcoated would be darker.
You may have all of the clearcoat off. Take a tiny dab of polish and try to polish a bit that you think still has clearcoat on it. If you get black stuff on your rag, you ARE down to the bare aluminum.
Today is a gift, that's why they call it the present.