Navy and Army Colts
After reading the above posts, let me comment that my study of collections of original frontier firearms has led me to the conclusion that just about anything was possible. For example, Colt and Smith & Wesson revolvers were converted to single-shot pistols (one shown herewith, from a collection in Santa Fe); instead, why wouldn't the gunsmith simply order another cylinder from the factory? In like manner, I've handled a 16" Volcanic carbine and a M1866 Winchester carbine altered to single-shot muzzle-loaders. In the same vein (and contrary to what I've read here), the EMF "1851 Pepperbox" does indeed have historical precedents. (See my post on that thread.) So it's certainly possible that someone hung a .44-caliber barrel and cylinder (from one gun or another) on a '51 Navy frame.