There are a million good articles about the 1851 Navy in books, and there are also some on the internet as well, so I plan to focus here only on things you don’t hear too often.
First let’s stop at the name 1851 Navy. I have to tell you that this terminology is a modern time invention. First of all the Navy revolver was not introduced in 1851 but in 1850 with the today so called 1849 Pocket revolvers. In fact the design was ready as early as 1847, but the company was overwhelmed with the .44 cal orders, so did not have capacity to start production. Colt did not have a separate patent for this revolver. He had one patent in 1850 but that is only about the safety rests for the hammer between the chambers.
Let’s get back to the Navy name as well. Colt originally used the New Ranger Size pistol” or simply „Ranger” name. When the The Army accepted the revolver for sevice they called it „Colt’s belt revolver” to differentiate it from the heavy Walker and Dragoon holster pistols.
Later Colt started to use the Navy name referring to the naval battle scene engraved on the cylinder. So what we call today an 1851 Navy revolver is a Navy not because the U.S. Navy used it, but because of this engraved scene. But the name lived on. Later the Army used the „Navy size pistol” terminology for all .36 caliber models from different manufacturers as well.
1851 Navy versions, according to the article by David Strout “The Colt Navy”
“The FIRST MODEL SQUARE BACK has the square-back trigger guard, brass back strap, and thin barrel lug, with the barrel wedge above the screw. This early model’s loading notch is the non-beveled V type, and the one-piece, wooden grips are the Slim Jim style with an upward flare of the butt on each side of the brass butt straps. The wedge screw is under the wedge, and the 5/16” wedge is placed through the thin barrel lug upside down. The serial number range is 1-1250.
The SECOND MODEL SQUARE BACK retained the brass square-back trigger guard, brass back strap with the screw placed above the wedge, along with the wedge entering up right. The barrel lug is thin and the cylinder pin is slotted. The serial number range is 1250-4200.
The EARLY THIRD MODEL has the small, round, brass trigger guard, brass back strap, and V-loading notch with the wedge screw over the wedge. The loading lever screw enters from the right. The serial number range is 4200 to 30000.
The MIDDLE THIRD MODEL has the small, round, brass trigger guard, brass back strap, V-loading lever, thick barrel lug, and the screw is over the wedge. The loading lever screw enters from the left side as do all the exterior screws, and the barrel lug was increased in size from 5/16” to 7/16.” The serial number range is 30000-37500.
Most of the LATE THIRD MODELS have the small, round, brass trigger guards and brass back straps. A small number are found with round iron trigger guards and back straps. The barrel lug is thick, the wedge is over the screw, and the loading notch is now beveled. The serial number range is 37500-85000.
The EARLY FOURTH MODEL has the larger round brass or iron trigger guard and the lading notch is beveled with a thick barrel wedge. The serial number range is 85000-118500.
The MIDDLE FOURTH MODEL retains the large brass round trigger guard and back straps of brass or iron. The loading notch is beveled with a thick barrel wedge, and the screw is over the wedge. The serial number range is 118500-16500.
The LATE FOURTH MODEL has the large brass trigger guard, a brass or iron back strap with beveled loading notch, and a thick barrel wedge with the screw over the wedge. The serial number range is 16500-215000.”