A bar of slate is a slate colored bar running through the red feather. Therefore, there should be red at the pin of the feather, or else it would just say slate undercolor, and not bar of slate. Tit for tat really though. If it's there all the way to the skin, that is still much better than...
Notice the backs in these two illustrations as well........Both the buckeye hen and cock have backs that go uphill. Hmmm. Interesting. Thanks for posting this as well, Laura. I love the old poultry books and this is the first that I have not seen before, on a certain web page.