Vintage Spectator Summer Dress

Is it a dress? A sleeveless duster? A swim coverup? Whatever it is, it's vintage and fabulous! Lightweight, seersucker cotton material in EUC. No size on tag, but I wear 4-6 and it fits perfectly. I am 5'4" and the hem hits me mid-calf. Looks amazing with a belt! See last pic for one small mark on back.