Beautiful Main Line dress from Thom Browne. Rare.

Beautiful dress by Thom Browne. Very rare. Purchased it at a vintage store in Tokyo a couple years back. Love the dress but I gained a lot of weight and cannot use it. I am not even 100% sure what size it is exactly. The tag says size 40. I think but it does not fit me. I am normally a small so I think this is probably extra small. There is a little spot on the dress which I have included in the picture.