Vintage YOUNG EDWARDIAN  black velvet dress 
Listed as Sz 11, but fits the model with a little extra room...
 she is US 4, so best for Sz 6 approx, but see measurements.
Chest area is boned.
back middle zipper
Acetate/rayon
Excellent condition.
Photo #10 shows you the elastic strap (close up)

36.5" long  (model is 5'9")
17" pit to pit.5 
14.5" waist flat
19" hips flat