It's a copy of a Colt 1848 Pocket Pistol often called the "Baby Dragoon" .
They came with and without a loading lever with 3", 4", 5" and 6" barrels and a 5 shot cylinder.
Colt built these from 1847 thru 1850.
Colt built about 15,000 of them during that time period.
Colt did some redesigning of this gun and presented it as the Colt Model 1849 Pocket Revolver with production beginning in 1850.
The most notable difference between the 1848 Baby Dragoon and the 1849 Pocket Revolver is the Baby Dragoon's square backed trigger guard, pretty much a small copy of the Walker and the 1st and 2nd model Dragoon.
The 1849 Pocket Revolver had a rounded back or "oval" shaped trigger guard.
The early "Baby Dragoon" cylinder had a Texas Ranger & Indian fight roll engraved on it while the later ones had a Stagecoach Robbery scene.
Early "Baby Dragoons had round or elliptical cylinder stops while the later ones had a elongated slot similar to the Colt 1851 Navy or the Colt 1849 Pocket Revolver.
From a Historical standpoint, the Baby Dragoon was Colts first successful return to the civilian market after the failure of his Paterson Company.
The success of this pistol and the 1849 led directly to Colts decision to make a larger .36 caliber civilian gun known as the 1851 Colt Navy.
With production starting in 1847, more than a few of these revolvers went West during the Gold Rush in 1848.
Even with its small caliber it could make, for the time period, a rather impressive burst of firepower in the civilian world.