Could be toning.
Have you measured the thickness to get a good comparison to a regular card?
If it is decently thicker, it could be printed on a piece of cheaper cardstock used to protect the actual stock being used from the pallet it would be on. That sort of thing would also be a typical thing thrown out and rescued and handcut.
The cheaper stock would also look different under a good magnifier. Brown stock sometimes has visible bits of wood fiber.
Steve B
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