T206 Ghost Overprint Back (Portrait)

The images showing above are T206 Frank Chance (Portrait, Red Background) with ghost overprint on card back. There are only two of these specimens with portrait overprint on card back known to exist, the other player is Jake Weimer.

These specimens have black ghost image of himself and it is the negative image of the card front. The negative black printing is in very fine details and it disproves the possibility of the black ink penetrate the cardboard. It also refutes the odds of the black ink wasn't completely dry during printing process and another cardboard stacked on top and the color transferred onto it.

One thing they are in common is they are both from the 150 Series and with Piedmont Back. If those black printings are the same as the card front, it could be easily explained. But they are the negative of the front images and in such fine details, the cause of these overprints may remain mystery forever.