I must be searching all the threads on this today! I have one like this and found a wiki about the slower feathering. The photos under the T/t allele section seemed applicable, and that they would be normal by 8-ish weeks. Scroll down the page a bit.
Wiki Link
My photos aren't as good as yours--black/barred chicks and in basement lighting. But what your picture is like is what mine are doing, but now three weeks old. If you are super curious there is another recently bumped thread where I posted them. It's on page two, not sure if this link will work right.
Other thread
'Wingnut' (such a good name, right?) is about a week behind at least on feathering in. But s/he is doing it. I read the males can be slower, but the discussion on the Wiki is regarding an autosomal, not-sex linked, mutation. Edited to add: the earlier section in the Wiki does discuss the sex-linked fast/slow feathering too.
My imagination is also telling me his/her barring is coming in more strongly. I will know in a few weeks I suppose! That said, it's a breed known for being easy to see gender differences at hatch (California Greys, a barred breed) and I supposedly bought all pullets. That said, out of 10, there might be one mistake statistically speaking.