Stunning! I am not sure, but it looks a lot like some of the 'altered couture' dresses made from antique and vintage crochet items, like parts of table throws, garments, etc. Possibly a one of kind, possibly not. I am not seeing 1920s or 1930s in this, as the bottom part in particular is too much like a large pattern, airy crochet tablecloth. As these crochet patterns and items were hugely popular and in everyone's homes in the Depression era, it would have looked odd for a woman to wear a dress made from crochet like that.
I could be wrong of course, lets see what others think. I have seen crochet dresses from the early 1930's, so possibly this is one.
Even if it is altered couture, it is very very pretty.