[Barty]

I had the same issue! My first k3 had a screen that separated from the bevel, so there was a gap there. So I got it replaced. My second was physically perfect, but it just did not look as good. Part of the screen was darker than the rest, like when you get a book wet and even after it dries, you can see the part that got wet is darker. The third one is okay, but it doesn't look quite as good as the first. The background is grayer. It kinda sucks but there's apparently quite a bit of variability with these screens. It's not something noticeable without side by side comparison. I decided to stick with the third.

You can call CS and just tell them the problem and see what they're willing to do for you.

Btw, have you tried to reset your first kindle to factory default and loading the latest firmware? Also, don't use any cover to rule out any such problem.