Atlas,
As I recall, there were few (or no) dummy boxes in the populated neighborhoods where I lived.
When my grandmother was buried in St Raymond's Cemetery, I took it upon myself to pedal to her gravesite. A long haul from Bedford Park, out Pelham Pkwy, White Plains rd, East Tremont Ave. My circuitous routes always included all the firehouses in the 20th Battalion (90/41, 61, 89/50).
There was an inconsistency to the box numbers out in what was then the hinterlands of the Bronx. Some even were missing. My dad explained that there were some numbers that did not exist as a physical alarm box on a street corner, but were a geographical reference point for the companies to respond to, as the Telegraph wires had not been strung to where the population was sparse.
That's what he told me about dummy boxes.
The big, tough, Irish SOB ain't been wrong yet.