Looks like all heartwood from the pics. In the last pic, there is green sandpaper to the left side and behind. Count down to the 3rd dark ring and try that one for the bows back. Remove all wood above it carefully and take your time. Choose thicker rings when you can. The white ring between each dark one should not be used as the bows back unless you put a backing on it. Seal the backs and ends soon if You're not going to reduce them all at the same time. Looks like so e good BL, nice score and good luck!
Tracy