swathdiver is basically correct.
Remingtons first army size revolver was the "Remington-Beals Army model", produced in 1861.
It's frame extended back so it almost touched the front of the cylinder and covered the barrel threads.
This led to the pistol locking up because of fouling after a few shots were fired.
The cylinder also does not have safety notches.
The Army wasn't impressed because of these two things so Remington designed the "Remington 1861 Army Revolver", also known as the "Old Model Army"** These were made in 1862.
These had a modified frame which was relieved to clear the front of the cylinder, leaving the barrel threads exposed. The sharp threads rubbed off the fouling on the front of the cylinder, thereby fixing the Army's biggest complaint.
Many of these had the safety notches on the cylinder added to please the Army.
This model also had a new idea incorporated into its loading lever.
Rather than blocking the cylinder pin when the loading lever was up, the lever cleared the cylinder pin making its removal easy for rapid reloading.
Too easy. It didn't take long for the Army to notice that a holstered gun could drop the cylinder pin causing the cylinder to drop out or fail to turn when the gun was cocked.
Remington fixed most of these by adding a small Fillister head screw to the top of the loading lever to retain the cylinder pin.
An estimated 6000 of these were made.**
The Remington New Model Army revolver was first made in 1863. It is the model that most (but not all) Italian reproductions represent.
All of these Remington pistols were built based on Beals 1858 Patent so although none of them were made before 1861 most people call them a 1858.
** Flayderman's Guide