@Cocktologist Welcome!
As others have said, @eje has done quite a bit of research on Hercules and published his own recreations on this blog, Savoy Stomp. Here is his recipe. Some people just use Punt E Mes as a substitute. Interestingly, The Savoy Cocktail book is the only cocktail book that has recipes made with Hercules. I am guessing this was some kind of clever product placement. 
In 2022, The American Bar at the Savoy brought it back, but there aren't much details other than it is a vermouth type ingredient that was recreated by this distillery.
