Hi Ted,
Maybe I'm not completely understanding the hypothesis. If you think the light Uzits were printed simultaneously with the light P42s, and the dark Uzits were printed simultaneously with the dark P42s, wouldn't you expect them to have the same front/back combos? But from your surveys, I don't believe Uzits and P42s have the same confirmed front/back combos.
Your confirmed Uzits are here:
Uzit Confirmed List
Your confirmed P42's are here:
P42 Confirmed List
As for whether the light vs dark was caused by truly different colors of blue, or was caused by how much ink was applied, I have no clue by looking at them. One way to objectively determine by scientific methods might be by analyzing the wavelength. I'm not really knowledgeable about this, but with your electronic background you might be. I'm thinking every color reflects at its own unique wavelength in the visible spectrum and absorbs all others when exposed to a white light. So, if the colors are truly different blues, they should reflect different wavelengths. But if the colors are the same blue but differ in how much ink was applied, then I'd think they should reflect the same wavelength but at different amplitude (intensity). Just a thought.
Best Regards,
Craig