According to the V&A, where this piece is housed:
This image shows the interior of a bodice, part of an outfit designed by Christian Dior (1905-57). The full ensemble consists of a skirt and bodice of lightweight organza, which float on a complex under-structure of net and boning in the bodice, and tulle in the skirt.What I like about this picture is that you can see the binding on the top of the corselet. According to my research, the inner foundation would often be bound at the top with a bias piece of the dress fabric, in case it peeked out. The corselet and the dress would have been tacked together by hand.
In-built corsetry was typical of couture garments, particularly for evening wear, so the client could simply step into her garment. In this case, even suspenders for stockings are included.
Anyone have a picture of this ensemble from the outside? The description says it has a fichu collar and a bouffant skirt.