Pretty early… or remarkably tardy, all things† considered! That Brucart recipe looks exactly to me like somebody hearing of a “Negroni” as Campari, gin and vermouth, and lacking a recipe, wandering his way to these proportions. (I’ll have to flip through the book and see if Campari appears anywhere else in it.)
I’m curious about the parallel history of Fernet. Fernet Branca obviously started exporting much earlier. The Fernet Branca Cocktail in the Savoy is a Negroni except for the different amaro. “One of the best ‘morning-after’ cocktails ever invented,” said someone (Craddock?).
Seems there was plenty of beating around the Negroni bush, just—as you explained—not the right circumstances for the concept to catch fire, except with a few with the ahead-of-their-time taste for it.
†All the random crap recipes that got rushed into the cocktail books…