Hi Peter, I will give you a very short outline of the printing process. Flat plate steel engraved printing:
The first step is done by an engraver. He engraves a small steel plate and once it is approved it is hardened.
Then the images is transferred to a soft steel transfer roll. The transfer roll if large enough can have multiple engravings rocked into it. once the image/images are rocked in the transfer roll is hardened.
The Transfer roll is then put into a press that allows the sideographer to rock the images into the flat steel plate spacing them in columns and rows.
In the case of the 5c on 6c, it is believed the transfer roll had both the 5c image and the 6c image on the same transfer roll. The operator laying out the plate is thought to have accidentally rocked the transfer roll too far when entering the 6c design. This caused a small portion of the 5c design to be rocked into the position for the next rocking of the 6c stamp. This is roughly what could have happened on position 21 in my pair.