They are very typical in early Bowman sets. They are wax stains from the sealing of the original package. Not sure about grading differences as I collect raw cards but I do know that higher grade collectors want very little if any on them.
They really don't bother me too much as long as I can still read the entire backs. I think they add character and is a throwback to why cards were printed I the first place and who they were aimed at.
Not sure how enlightening all this is but maybe it helps a bit.
Drew
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Drew
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