The pattern calls for a petticoat--obviously not the 18th century kind. I found this vintage black one on Etsy.
I also found vintage gloves and a beaded little purse at an antiques store for about $12.
Made by hand in Belgium!
I recruited my 60s sewing friend for help deciphering the pattern and cutting it and constructing it.
I noticed that in the Newport Art Museum's book, Newportraits, Mrs. Elinor Ingersoll's 1956 portrait by Channing Hare shows a curved seam at the bust just like my 1956 pattern.
...though I also found those curved seams challenging.
Unfortunately I didn't finish this in time for my deadline. It needs a zipper and corrections in a few spots. It has to wait because it's time to get ready for The Challenge!