Unfortunately, not universally accepted.
I understand where Larry is coming from. As one who has most certainly closely examined more era photos of the first half of the 20th century than have 99 percent of the educated people of the world, and I have closely examined literally tens of thousands of era photos of model T Fords. I have seen too many restorations done wrong on simple details that could have been easily researched. I have seen many 1913 and 1914 model T Fords with their horns mounted upside down. What is right and what is wrong has been debated for many decades, with the whole "bell up" and "bell down" thing confusing everyone that expects a simple "one size fits all" answer. The fact IS that bulb type horns were used for nearly two decades on automobiles and manufactured in literally hundreds of models and sizes. Many, actually far and away MOST, had some type of twist to the horn (which amplifies the sounds created by the reed vibrations) in order to maintain a practical size. By twisting or rolling the nearly two to as much as four foot long bell/horn it became an easy to place six to eight inch size.
Horn "twists" could be "right hand" or "left hand" twist, and the choice altered where and how the mounting bracket might fit the - - - whatever it was mounted onto, as well as how the longer horn tubes needed to be routed due to offsets in the connector. And, of course, everything was further complicated by the fact that a lot of cars were right hand side driven unlike the model TS of the USA. And some cars were sold without horns or people wanted something different or a replacement for a damaged horn providing manufacturers with countless opportunities to sell after-market horns.
The 1913 and 1914 model T Ford horns were one of the most common style ever in production, manufactured by several different companies! And guess what? They were manufactured in both right hand and left hand twist versions (not even taking into account the Canadian right hand drive cars!). The model T replacement was common and cheap, and a lot of people did not notice or care about the direction of twist.
The simple fact is that a lot of people restoring 1913/'14 model Ts put them on the wrong way. A lot of people wind up with Ford looking horns that are twisted the wrong way? But they make them fit and work. And a lot of people have correct Ford horns, but they manage to mount them upside down and still work.
Now, about those era photographs. I know that over the years, I have seen a few era photos showing 1913 or 1914 model T Fords, clearly showing the horn what we consider to be upside down. To further confuse the issue, many horns on earlier model Ts were in fact mounted "bell up" in such a way that the if water were to be splashed into the bell, the water would right on down into the coiled horn tube. Many other cars of the era did also mount their horns that way. As far as the industry was concerned at that time? I seriously doubt they spent much time debating the right or wrong of it.
Although I have seen many 1912 model Ts with the horns "bell up", actually, with the horn tubing run outside the body, "bell up" actually made it simpler to put the tubing down low enough to be out of the way. By 1913, Ford put the tubing inside the body, necessitating the tubing being above the floor, and with a fake door, lots of place for the tubing to go. 1912 had a wider variety of body styles, some with different horns, as well as changes in the horn through the year. For 1913 (and on through 1914 and into the early 1915s), for basically the first time, Ford basically standardized the horn and it's placement on all cars.
However, earlier today, I decided to take a few minutes, and take a quick run through some of the era photos on my computer. To be clear, it was about 150 era photos of 1913s, and I lost count around 200 era photos of 1914s. Out of all those, I did not find a single 1913 with the horn "upside down"! I did find one 1914 T stuck in high water, ironically trying to show why the horn should be mounted so that the water doesn't enter the bell and run down into the coiled tube.