I have a couple of things to contribute here. One is that as a teen in 1971-72 I saw and handled a "Coach Gun" with Wells Fargo engraved on the midrib. Double barrelled, Damascus twist and hammers. For the life of me I cannot remember if it was pin, centre or rim fire, I vaguely think pinfire... and very short barrelled. My small town, Shrewsbury UK had 5 gun shops, and two of them were run by people who also collected, and this was in the one that was run by one of the more enthusiastic collectors.
Regarding Doc's gun, I remember an article in "Guns and Ammo" where they reckoned he carried a sawn-off DB, both ends cut down to 12 inch and a pistol grip, on a strap under his coat and balanced so that it levelled when he threw his coat back. Apparently it was a copy of Mormon Bodyguard, Porter Rockwell's piece. (I have another reminisce that I can share about cut down shotguns, but don't want to clog up this thread.)
https://en.wikipedia.org/wiki/Porter_Rockwell