As Russ T Frizzen said the silver backstrap and triggerguard are correct for civilian 1851's.
This is also true for Colt Navies made in the 1800's.
Over time some modern replicas have been offered with silver backstraps and triggerguards. If I recall correctly several very early 1851 Navy Arms I owned had this feature.
In the 1970's at Simm's Hardware Store in Sacramento (Simm's was a Colt warranty station) I saw a letter from Colt to dealers which stated that tarnish on the silver backstrap and triggerguard of the 1851 Navy was not a warranty problem. :shocked2: :rotf: