A few more I've picked up over the last 40+ years. 
Many years ago, when I was running a gun shop, a customer brought me an old percussion revolver, double holster rig and powder flask to see if I could identify what the gun was. I was an expert on antique firearms and the gun buyer for Simms Hardware and later managed the Old Sacramento Armoury. In any case, when he pulled this huge percussion relvolver—that a friend going through a divorce had just given him (!)— out of his gun case, I thought immediately "Oh my God, it's a Walker!"
Then I took a second look. . . and realized it wasn't a Walker at all. I thought perhaps it's a First Model Dragoon, but no, it's not. Certainly not a 2nd. What in hell is it?
He left it with me to research what the heck it was. It took me several weeks of digging but I found that what my customer had been given was a Colt that was a gun that was rarer than any other production line model Colt ever made. Only 337 of them were made and they had their own serial number range at the end of the Walker range. They were made on Walker frames and parts with First Model Dragoon barrels and loading levers for the specific purpose of replacing the Walker Colts the Texas Rangers had lost or destroyed in service! They'd been considered apochryphal as the records had been destroyed when the Colt factory burned in 1862(?). But in the 1930s a collector by the name of Fluck did deep research and found duplicate records documenting the order along with invoices in the Texas Ranger Archives specifying the changes and the run in a letter from Colt, and established the model and its existence, made prior to the release of the First Model Dragoon. They are called the Colt Fluck Dragoon in that collector's honor.
It's likely that a total of 1,000 Fluck Dragoons were eventually made by Colt using up Walker parts, selling the balance to the public. They are still the rarest production, non-special order gun made by Colt.
At the time my customer brought this one in, there were 26 known surviving examples in collection and museums. This was the 27th found! There have a couple more turn up since. About five years before then a rusted, beat up Fluck in ratty condition sold at auction for $17,000 Plus buyers premium of 15%. This on my customer had was in really good, shootable condition with about 20% original finish. Probably worth then a good $30k. The holster rig was even rarer, being an original Walker saddle holster for two Walkers. . . and few of them have survived. The flask was an original Colt Walker powder flask, also exceedingly rare! Estimated value of the holster was $15k to $20k, flask around $5,000. Some gift!
By the way, prior to the Walker in this auction turning up, there were 184 known surviving Walkers in museums and private hands. This B Company serial # 45 was not catalogued among those, and makes the 185th.
A few more I've picked up over the last 40+ years. Thank you for that great information! I had no idea the “Fluck” models even existed.
Cool! Thanks for telling us.