Though the film is officially based on the novel "A Shilling for Candles" by Elizabeth Mackintosh (writing under the name "Josephine Tey"), Alfred Hitchcock and his writers only used about one-third of the novel and changed the identity of the murderer.
The failure of the original copyright holder to renew the film's copyright resulted in it falling into public domain, meaning that virtually anyone could duplicate and sell a VHS/DVD copy of the film. Therefore, many of the versions of this film available on the market are either severely (and usually badly) edited and/or of extremely poor quality, having been duped from second- or third-generation (or more) copies of the film.
Though the viewer finds out at the end who the murderer of Christine is, the name of the murderer is never given in the film. Even where Christine is shouting at her eventual murderer in a long conversation, she never names him, and the drummer at the end of the film is jokingly called "Old Man" by his fellow musicians, not by his real name. In the original story on which the film is based ("A Shilling for Candles"), the character has a name--Edward Champneis--but it is never given in the film. This actually would not be a problem, as the name of the murderer isn't important, except that the film credits (not just the IMDb credits, but the actual credits on the film) erroneously give the character (played by George Curzon) the name "Guy." No character of that name appears in the film.