Huh. Never really thought about the history history. I just pick up whatever collection is at the used bookstore. Makes you realize how much work goes into keeping these old stories in front of people.
St. Clair’s established habits are a clue. His wife says he would often go into the city on business. That regularity provided the cover for his double life. The mystery begins when he breaks that routine—he doesn’t come home. The breakdown of the normal pattern is what triggers the alarm. The solution involves reconstructing what that ‘business’ really was: a daily performance. The clue is in the mundane schedule that allowed the secret to flourish for so long without suspicion.
Compare it to other Holmes twists. Often, the villain is a sinister figure with a complex plot. Here, the 'villain' is a weak man with a simple, grubby scheme. The twist is almost anti-climactic in its simplicity, which is why it's so effective. After the build-up of a possible murder in a den of iniquity, the truth is small, sad, and greedy. It deflates the melodrama, which is a bold and interesting narrative choice.