One queston, isn't 15 inches the point to start the armhole shaping? The shoulders start at 15 + 8 inches?
At any rate, looking at the photo of the back, it looks like the horseshoe and lace pattern (9+15+9) flanked by the lace pattern so that you have 15+9+15+9+15 (total 63sts). You could do stockinette on the ends (4sts each side) to add up to 71. That's what the photo looks like to me.
I'll take another look at it but that's my first thought.