I'm not sure if this is a gum or a wax stain, but I have several other '67 Topps cards that have more or less the exact same thing.
Question is, if this is an EX or EX+ card otherwise, what does this stain do to the grade in your opinion? Does it drop it down to VG-EX, or worse?
The good thing about it is the stain is mostly clear, and when you are just looking at the card straight on, not at an angle in bright light - it's very hard to see.
Card looks like this when not twisting it in light looking for the stain:
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