I think that all nine of mine are finally laying, as of yesterday! The girls are 27 weeks old. I had five eggs in nest boxes, and I knew who laid four of them. So there was one mystery egg in a nest box. Then there was the egg laid outside on the deck to their coop, and the egg laid while roosting early in the morning (it fell and was smashed).
So four known eggs, and three eggs I don't know who laid them. But of the five suspects, two of them have been laying for a couple of weeks, and lay much darker eggs than the unknown one in the nest box. The one on the deck was dark, but the color wasn't uniform, and the egg was a little misshapen, like a first egg. And I don't think the two that have been laying for a couple of weeks would have laid one while roosting. So that means the three I didn't know were laying must have laid those three eggs!