Personally, I highly doubt it was a deliberate change at the time of printing. It's too insignificant of a detail to have been altered, especially with the stone age printing equipment that was used at the time. My guess is that the spaces between the teeth were such a tiny detail that they came and went depending on the amount of black ink (and possibly other colors) on the plates and how good everything was aligned during the printing process. The 'black tooth' versions were most likely the result of ink pooling in that area, because the amount seen there seems to vary pretty greatly.
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