1922 H.R.H. Princess Mary post card
From eBay that is also advertising a 1922 issue of Country Life showing her wearing the same dress.
Alice Hughes shows Princess Royal Mary wears an evening dress without a separate waistline and a vest-framed upper bodice in this photo.
Keywords: 1922, Hughes, Princess Royal Mary, Windsor family, Princess, Countess, British, curly coiffure, head band, over-bodice, elbow length loose sleeves, tight skirt, necklace, bracelets, wrap