Is the back glossy at all?
Cardboard differences aren't unknown, but I haven't really tried finding them.
There's also some differences in gloss for a lot of cards, especially Topps in the late 70's early 80's where some gloss stays clear and some yellows slightly. I'm not sure if it's an actual difference, or a sped up degrading maybe from something like the cards sitting in a hot car or something.
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