Start a new thread @what is value as this one is 10 years old. Post photos of both sides in full frame, not thumbnails.
Go to forums and pick the one that applies. Start a new thread will pop up so hit that button. Post full frame photos and state what you see or ask questions.
Follow the stages pictured below Choose the forum you wish to post in, then click on the blue Post New Thread button. Type the title of your thread in the box and your question/ observations in the text space underneath. To add images scroll to the bottom and click on Upload a File. When each image is uploaded click on the Full Image button. When all your photos are there click Create Thread. That's it
Most likely Environmental Damage. Exposure to the elements tones the Cupro-Nickel clad layers to a dark brown color. Not a mint error.