Ive made an animated overlay with another 1970 cent from Philadelphia for the year..small and large die variations were only issued from the San Fran mint they switched the die sometime during the minting process. That being said...you can see where there is a line in the 0 that maybe angles towards the same spot in the 7 at the lower section...that may indicate a strike through error? something like a staple or something maybe somehow made its way into the press, but I really dont this that is the complete answer? Can you do me a favor and send me another image of the error? As large and clean as you can?