I don't have your resources. What I see are two coins from two emperors. Leaving top and bottom characters aside, I see three characters as the same (left and mint) and one different (right). The difference in the right character is subtle (top of character), but you have sensitised me to such subtle differences. Also, the difference is notable in context:
At first sight, the sameness is the strangest. When the new emperor ascended the throne, there was an urgent need for new coins. I am assuming the new mothers could not be prepared in advance (maybe the emperor's reign name would not be known until he was enthroned). Still, the change could be prepared by creating a grandmother with only the name of the emperor left blank. As the name became known, two new characters needed to be engraved. If you are a control freak (Chinese mandarins usually are), you want something to recognise the transitional coins by. You make a small change to t'ung and presto!
What baffles me is the character pao, though. To achieve such sameness, you cannot trust the engraver's hand. You need a mechanical process. Here is how I imagine that process. The key is that the BOR (North) does not use the grandmother that produces all the mothers, because it is holed up somewhere in an unspeakable bureaucracy and it would be bad luck to think of the emperor's death, even when it's imminent, so the grandmother is not released. I imagine the BOR using its mother. It is used to make a pretty good child. The child is handed to an artisan, who fills the characters of the name of the emperor to turn them into a big blob and makes the change on the t'ung character. When the new name is known, he turns the blob into the new characters. This piece is used to make a new mother, with no trace of filing left.
There are problems with the above scenario. One is that it assumes all (grand)mothers are cast. It would have been easier to work with a technique that produces alternative incuse and relief characters. You could just file the old characters away, produce an incuse and sink the new characters. Another is that it produces just one seed coin. A third is that it may be over-interpreting innocent similarities.
Comments? Bruce? Anyone?
Peter