Actually, all my chicks are 6 days and all of them have flat areas. The one's that are a bit older have combs. I think it takes at least a week to two for combs to start to develop. I also googled pictures of chicks and it seems that very young ones are all flat.
I've watched day old chicks hatch out with their combs obvious before their feathers are even dried. Not even large combed breeds. If a bird has a single comb it will be there even as young as 1 day old. Tiny, yes, but visible. If your birds have all flat combs, they are likely pea or rose combed. Based on the coloration and Greenfire's website, I'd say you have Marsh Daisies, Twentse, or Wyandottes, or possibly all three.