So here is one back piece. Compare it to the right side of the diagram in the last post. M. de Garsault says the tops of the pleats need to be re-enforced first. I made mine out of scraps of brown hemp canvas. The coat back has two: the rectangle on the left keeps the slit above the center back pleat stable. The horseshoe-shaped bit is partially covered by the hymo that lines the pleat.
Next time the fronts get their interfacing.