I've been looking at that today on your blogspot Del, that's what gave me the idea to do this one.
It looks like an old crack, (maybe always been there, I can't think how else the edges could of oxidized) because the edges are dark and have always been covered with a thick plastic sticky wrap and sticky gaffer tape under the leather wrap, it looks like its pinched at the south end of the crack where it goes off at a right angle and shows in the varnish, the north end of the crack looks stable as its end has a brown edge. Its not a splinter and the back seems sound. Here's a pic, its the opposite side of the arrow pass. That area is mottled darker wood so presumably remnants of a knotted area it was cut fromso a harder wood area, i tried another pic close up. (The lines running across the centre of the crack are glue remnants of the sticky tape.)
Bottom limb is facing south in the pictures and the back is to the right.