Hi,
I have many 1969 Topps Football that have that outline of the green football that you describe. It is especially noticeable if you tilt the back in the light. During the 1960s and at least into the sets issued in 1970, Topps used 2 different card stocks- the more common stiffer cardboard and a softer more bendable whiter stock. The black circle is very obvious on the softer stock cards, and I believe that it is actually green ink from the football's imprint. It appears that Topps enlarged the imprint of the football instead of trying to exactly match the green to the football design. Since the back was black it isn't that obvious except on the softer stock cards.
I hope that this information helps.
Jeff
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