OK, the mystery designer is Ben Barrack.
I know some of you are going, "Huh, who's that?" Or, "He did that dress?" Actually the dress does have a lot of his signature styling, but you wouldn't think so if you're familiar only with his 60's work. He's without a doubt, my personal favorite--the fabrics he used and the meticulous construction are second to none. And, his early styling was beautiful. Because his was such a small house and he apparently was not as prolific as many of the designers guessed, he's not extremely well known. But most people who are familiar with his work really like it. He designed for his own house and under his own label for B. Altman, I think, or it might have been Bonwit Teller. I forget.
I think there are a lot of designers like him who should get more recognition. If you google for his dresses, you'll find some lovely examples on some web sites!
And, laughing magpie, Jonathan Logan was a good guess; you're right--they're early stuff was very stylish, and I've seen a lot of full-skirted dresses from them. I love their 50's stuff!