She owned the tiara already in 1953, wearing the diamond flower as brooch on her fur toque during the wedding of HRH Princess Joséphine-Charlotte & HRH Hereditary Grand-Duke Jean.
In 1958 she wore it during the last & most famous courtball.
But I have seen a video once, Lilian was wearing another gown. Lilian was on that video together with HRH the Duke of Edinburgh.