I did some testing on a scrap piece of my HHB (I had an 80 inch stave, only looking to make a 68 inch bow). I am seeing the dark cambium, the ridged white, and then smoother, whiter wood beneath that. I think the ridged white is the back that everyone is talking about! Which I still have on my stave!
Here is my stave before roughing out and after getting the top limb bending on the floor. I picked a tough stave to try as it had the least wood total and I needed it to dry quickly!