We have 2 sebrites, and I seem to remember them being very wee, even at that feathered stage. I was going to guess silver laced wyondot. We have one of those (adult), but no photo handy.
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My first guess was Silver Sebright also, but looking at his size I think it is the other breed you mentioned! I have 27 2 week old sebrights, and they are WAY smaller than yours!