Del, I'm still working on my first cup of coffee, so I may not be following what you asked. Hickory, like most white woods, still has a dark center, and as Dean mentioned the heart wood can vary some in both degree of darkness and size. Depending how this bow/stave was cut at the saw mill it might very well have a percentage of the board heartwood. Although hickory sapwood is recognized as slightly better than the sapwood I believe that wood bows made from staves that the heartwood is still the belly of the bow and the sapwood side is the back (at least this is how I have always worked it). What I find intriguing is that the bow you looked at is concaved 4 inches toward the darker side (?). As you know a stave while often warp slighty toward the bark side (the lighter color) when drying--maybe it was cut from a warped trunk?