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I believe that instead of the Colt Paterson revolver (which has a barrel with 6 flat sides), that this based off of the successor to the Paterson, the Colt Walker (which has a round barrel exactly like Cad's). And since this is actually the pistol used in Good, Bad, and Ugly (as shown above) I think is it.
And as SW has done so many times, they have modified from the original. (It's obvious from the images from the episode, that Cad's pistol is not a cylinder revolver....so if you cut the entire cylinder section off of this gun, move it back, and add a small raised block onto the flat section, and then remove the trigger guard, I believe you would have Cad's gun exactly. I love scratch building, kit bashing, or modifications to get to a new/original piece just like the propmasters at Lucas do. I'm pretty sure this is what they did.