This gown, made for Janet Leigh for That Forsyte Woman, eventually went on to be used in at least two other films. It finally went up for auction. You can see the listing here, which describes the dress as:
A 19th century style golden silk floor-length gown with lace trimmed decoration, designed by Walter Plunkett. Stamp marked "MGM" and with a handwritten inscription, "1435/ 5623/ Janet Leigh." The film number on the costume tag "1438" indicates that this was made for Leigh for the film That Forsyte Woman (MGM, 1949).
The dress has clearly seen wear and tear over the years, and more than likely has been used in many other films than documented above.