is that they appear to have concave spots??

As I understand it, very few casinos used dice with concave spots, because it put the dice out of balance. I mean, the amount of material drilled out of the 1 side is much less than the amount of material drilled out of the six side. So (with concave spots)....the one side should be heavier, and more prone to land on the bottom.

At some point they came up with a "paint" (filler?) that weighed the same as the base material of the dice...so they filled the spots flush, and the dice would be balanced.

Here's some cheaters with a casino name, and flush spots

Look in the mirror...you can see that they have the same number of spots on the opposite sides!