I don't believe that the reverse Piedmont impression on the Nicholls card posted above was a result of the printing process. It most likely happened during the storage of the card.
Many different examples of cards have been shown with storage transfers that are mistakenly called wet sheet transfers or thought to have occurred at the time the cards were printed. In most cases the ink from one of two cards stored together will transfer to the other. The exact variables that cause this are not known.
The Nicholls card above has a similar effect but slightly different. Instead of the ink from one card transferring to the other, the ink from the Piedmont back protected the blue sky of the Nicholls card. Where the heaviest portions of the Piedmont's blue ink pressed against the Nicholls the sky did not discolor. The areas of the Piedmont back that had little to no ink did not protect the sky from discoloring.
If it was a result of printing more of the Piedmont back would be visible in other areas of the Nicholls card.
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