The "American" 1851 Navy had silver plated Brass triggerguard and Backstrap. The London Navy had an iron backstrap and triggerguard, as did the 1861 Navy, and the Colt SAA. The Colt SAA gripframe was the same as the 1861 Navy.
The Italian made repros use steel for the London and 61 rather than iron.
Not exactly. That is the 'conventional wisdom' about Colt's Old Model Belt Pistol of Navy caliber - AKA 1851 Navy. The real story is somewhat different.
The American 1851 Navies made through the late 3rd models had brass BS/TG. The late 3rd models all of the 4th models could be had with polished brass, plated brass, blued iron, or plated iron BS/TG.
All of the Early London models - through about SN 1396 - were made from parts shipped to London by Colt for assembly and all had Brass BS/TG. The London-Londons models, those that were completely manufactured in England, had Iron BS/TG. Finally, the Hartford Londons, those that were manfactured in Hartford for the London market, had a mix of Iron and Brass BS/TG.
info from '51 Colt Navies by Nathan L. Swayze