I only stuck the Mazarin diamond case in at first because I vaguely remembered the name as intriguing - and then I re-read it, and there's the waxwork of Holmes (which also turns up in another of the stories), and I realised that Sherlock needed an updated version of that. I have a feeling that 'Billy' may end up appearing in other stories as well in the future, he's just too handy a prop to waste.