Hmm... That's very puzzling. I've seen it before on similar condition coins, but never stopped to think about what it is. Sorry, but I can't really help you on this. Let's see what the others think about it.
It happens on worn out dies the repeated strikes of certain die pairs, will start to show the image from the opposite side of the coin. It is called progressive indirect die transfer. http://www.error-ref.com/?s=Progressive+indirect+die+transfer
