The special effect used for when the ghost appears was achieved by using two identical sets and a pane of glass angled in between them. This technique is called "Pepper's ghost," named after the English scientist John Henry Pepper (1821-1900) who popularized the effect with a demonstration at a theater in 1862. Teleprompters are a modern use of this technology.
Sylvie et le fantôme (1946) is included in the "Eclipse Series 45: Claude Autant-Lara --- Four Romantic Escapes from Occupied France" DVD set, released by Criterion.
At time of production, Odette Joyeux was 30 years old. Due to her petite figure and height, she was reasonably passable for a girl half her age ready to celebrate her 16th Birthday.