Naderlander
Well Heck. To speculate. One has to take one's gun apart and send to the parts to be Nickel Plate (DIY is a Nuisance), and while the subject parts are on hiatus one cannot play with one's toy(s), which is also a nuisance. Generally speaking it's just a pain to have done. Of course, it does make good sense as pretty guns do shoot better.
In past sample examples, Armi San Marco use to Silver Plate the back strap on their conversions (not all) which looked really good. Because of that, they shot really well too. Along with that, Pietta offers an 1862 NYMP Percussion gun with a snazzy Nickel Plate back strap. Even has a nifty lanyard ring.
Now, in all honesty, I have no clue how many, why, if, when, etc., Colt were shipped with Silver Plate back straps. Never cared. Not my cuppa coco. Still don't care. I have from time to time, been tempted to ship off a back strap or two for Nickel Plate but as mentioned above, I wouldn't be able to play with that particular toy, so skipped it. Oh Well
Remember: People Are Still Hazardous To Yer Health. Avoidance