Repeated references are made to "oil of vitriol." This is the old name for sulfuric acid, in this case thrown in the faces of women to disfigure them.
The prop book of Baron Gruner's affairs is full of detailed records thoroughly written in correct German language.
The soundtrack includes several references to Mozart's opera Don Giovanni, in which the lecherous Don Giovanni is punished for his misdeeds. When the Baron is examining his book, he listens to a wax cylinder gramophone playing the Madamina aria in which Giovanni's servant boasts about the huge number of women his master has seduced. Then the final confrontation with the Baron is accompanied first by an excerpt from the overture, and then by the music from the scene in which Giovanni is finally dragged down to Hell.
Although Conan Doyle himself never expressly identified who the client was, this episode leaves little doubt as to the identity when Watson sees the coat of arms on the outside of the carriage that Sir James climbs into at the end of the episode. Earlier Sir James admitted to Holmes the identity was a "he", so the coat of arms confirms it was in fact King Edward VII himself.
Kim Thomson, who played Kitty Winter, was cast in this episode after playing Moriarty's mistress in a TV movie called Hands of a Murderer with Edward Woodward as Holmes and Anthony Andrews as Moriarty.