Great investigative work Steve. I had read that exact same passage about Rubel before which is what lead me to believe that it was offset printing. But combined with the evidence from the Willett, I would say that you have hit the "nail" on the head.
With regards to the ghost underprints, since your theory is that the plates would have been the reverse image then:
- An error occurred with the feed, and a sheet didn't get sent through which caused the ink to transfer to the rubber impression roller (unintended).
- The next sheet that gets fed gets it's normal image as intended on the front, but the rubber impression roller also transfers ink to the back of the card. The back image is reversed in the same way that offset printing works. Plus, this is why the back image is positionally (although mirrored) in the exact spot as the front. It was a single feed that transferred ink to the front and back of the sheet simultaneously.
- The sheet that followed that may or may not have the ghost under print, depending on how much ink was left on the roller (perhaps why the Quillen I just acquired is SOOOOO faint).
Great work, Steve. Just great.