Would an original Colt 1851 (1856 build) have a steel trigger guard? The barrel is marked U.S. America so I dont think this was a london production. By the serial number it shows up as built in 1856. Or is this some sort of reproduction?
Colt closed the London plant in 1856 and shipped unfinished guns and parts back to the US (he kept the sales office open but seemed to have determined that demand could be handled by the US plant). If your gun was manufactured in 1856 it's quite possible your gun could have been built using some of the leftover British parts depending on when it was made during the year. Other guns have been found over the years with British parts on them and are documented in various collections. Sam Colt wasn't about to waste perfectly good parts.
The 1851 Navy series normally has a spring on the wedge, The various pocket models (1849 .31 cal., the .36 cal. 1862 pocket police and pocket Navy) usually do not.