From the Museum of Fine Arts.
It could be that the robe volante, like the 1730 example seen here, blended with the tontillo became the robe à la française with the cut, including the back panel from neckline to hem coming from the robe volante and the support structure and wide narrow shape coming from the tontillo. The supports of the robe volante made for a round skirt more like the crinolines a little over a century later.
Keywords: 1730, French, vee neckline, full elbow length sleeves, cuffs, full skirt, robe volante