Just took a look down the barrel of my Mountain Pistol. The answer is yes, it does have the through breech plug drum. Start soaking that stuck ball with Kroil or your favorite penetrant oil. You are probably going to need penetrating oil on the drum as well. Pull the nipple and start soaking.
Thanks. Kroiling away. The bore is really a mess. I found a possible source for a NOS replacement .45 barrel, and may go that route anyway. My son has a US made Mt. Rifle in .45 and it would make a nice match for him.