The genealogical article for Anna Josepha Dietrichstein née Khevenhüller is here. She lived from 1705 to 1764. Her husband’s Wikipedia article is here.
From restaurovani-textilii.cz/en/gallery/; removed electrical fittings to the left with Photoshop and enlarged one quarter. Also posted to the Reign of Louis XV - 1715 to 1774 Album.
This dress has a sacque back, but the skirt and bodice merge almost seamlessly with the serpentine ruching. The stomacher was probably showier than that seen here and she sported full engageantes when she wore it.
Keywords: Maria Anna Josepha von Khevenhüller, von Khevenhüller family, von Dietrichstein family, Gräfin von Khevenhüller, sacque-back bodice, vee décolletage, revers, elbow length close sleves, flared cuffs, stomacher, bows, ruched skirt