Your serial number indicates construction as a Model 1849 between 1852 and 1853.
The squareback triggerguard was dropped from the Model 1848 at the same time as the overlap with the Model 1849 production - at about serial number 15500.
Serial numbers for the Model 1849 began at 12000 - in 1850.
Parts could and did overlap during construction - but there's no way to discern just how often this occurred.
"7. Among the innumerable variations of the Model 1849 are the following: varying overall rammer size, lever latches, lever catches, iron backstrap and triggerguard types, rammer lever assembly screw, grips, quality of silver plating on grip straps, hammer spur knurlings, crowned muzzles, British proofmarked specimens, wedge keys with restraining springs, 36 CAL triggerguard stampings, lack of barrel or frame stampings, dealer stampings, and even variations in the cylinder roll engraving. Most variations in the Model 1849 appear in the period 1850-1860."
For more - look at Sutherland and Wilson's book - 'The Book of Colt Firearms'.
There are folks out there who collect 'just' the Model 1849 - because of all the possible variants.
Vaya,
Scouts Out!