David Weaver
..may not be out of flat in a way that's favorable to you. For example, I've gotten a lot of older razor stones that were hollow in their length, but the barber who had them would've had razors that matched the gradual hollow on the stone. If another person put their razor on it, the result may not be great.

Whatever you do with your knives, it's going to go in the shape of the wear.

Typical for me to see with older stones is a little bit of hollow in the center, never more, but often a lot of hollow in the length. I'm sure that whoever is using the stones is moving their plane irons on the diagonal and then working the back of the irons over the edges of the stones. It can be done indefinitely without much degradation in the quality of the edge.