It was black and strapless, with a sassy sequined flounce at the bodice and a gloriously full, swishy skirt. The dress was, Lady Diana Spencer thought, so grown-up, just right for her first official engagement after the announcement she was to marry Prince Charles. The dress had been missing for years until David Emanuel recently discovered it in a plastic bag at his home. Along with other garments worn by Diana and designed by the Emanuels, it is to be auctioned off June 8 in a sale that includes the silk chiffon blouse chosen for the Princess of Wales' official engagement portrait by Lord Snowdon, and the calico prototype used to fit her famous ivory wedding gown. Prince Charles reportedly didn't like the ballgown she wore to the charity event, because he thought black was for people in mourning. Diana thought it was tres chic, and anyway, she had nothing else to wear. “She was unsophisticated at that point, and when I look at the wedding dress and the black ballgown I can see a young girl's dream of the ultimate party dress or romantic dress,” said auctioneer Kerry Taylor, whose eponymous firm is handling the sale. “So there's an innocence about these early pieces. The black dress is expected to fetch between $44,000 to $73,000 and the prototype of the wedding dress between 8,000 and 12,000 pounds ($12,000 to $17,000).