The reason that Remington doesn't show the barrel threads like the New Model Army is because that pistol is not a New Model Army.
It a reproduction of the Remington 1861 Army Revolver, often called the "Old Model Army" in it's late stage of production.
This pistol used the original Remington-Beals frame which did not have exposed threads at the rear of the barrel.
The early production of the 1861 Army did not have the safety notches at the rear of the cylinder and it used an odd loading lever with the top relieved to allow removal of the cylinder without dropping the loading lever.
Shooting tests conducted by the Army found several faults with the Old Model Army. One was because the cylinder pin could be removed without dropping the loading lever, often the pin would work itself free and allow the cylinder to fall out.
Another fault was there was no way to safely carry the pistol with all 6 chambers loaded.
Last but not least, There was nothing to scrape the face of the cylinder to remove fouling. This would cause the gun to jam up making it inoperative.
To address these issues, Remington replaced the loading lever with its cut away area with the earlier Beals style. They added safety notches to the rear of the cylinder so the hammer nose could engage them to make the gun safe to carry with a fully loaded cylinder and they enlarged the area where the cylinder is to expose the barrel threads. By exposing the barrel threads, the sharp edges scrape away the fouling on the front of the cylinder.
Remington called the redesigned gun a "Remington New Model Army" revolver.
