Just for what it's worth, i heard this funny story today from an engraver at VSL, and we ended up talking about these PU signatures.
i have always thought that it was a special design, slightly different from the registered one, which makes it different from the rest and quite special.
He tells me this...imagine an engraver at work, and suddenly, for instance he has to sneeze...and by accident moves the item he's working on, in the wrong direction, which ends up in an error in the design.The piece then had to go back to the drawingtable, to have the design slightly altered or redisigned to hide the error.
So once again, it is different from the registered design, which makes it sort of unique, but not by choice!