Want this question answered?
Cabella's
there are several for sale on gunbroker.com and auctionarms.com you will need to know the maker and caliber of your stock to get a proper fit.
Capital I within a box is made by Interarms.
It's on the barrel under the forend. You'll have to remove the forend.
Cabela's sells some double barrel muzzle loading rifles. Pedersoli sells some double barrel muzzle loading pistols. You could also go to GunsAmerica.
Impossible to answer without a DETAILED description of all markings, barrel length, finish, type of sights, wood, condition, box, papers, accessories.
The Thompson Center hawken that I have,has a barrel that is 28 inches long.
I think it's .38 super. The caliber should be stamped on the barrel. Any gun shop or gunsmith could tell you if you took it to them.
The 19th century muzzle loader was quite inaccurate. This is because there is no rifling in the barrel. Rifling is the grooves in the barrel that cause the ammunition to corkscrew and the bullet fires straight and true(er).
My .22 caliber mercury rifle has a stamp on the barrel of a "crown" . which is usually a sign it was made in the UK somewhere, my 308 has the same stamp of the crown on it and was bade in Birmingham England
Depending on the maker you can remove the breech plug and of course the sights and underlug. It is not an easy task to remove the plug and without proper tools usually results in a buggered up barrel or plug. You would need a barrel vise and a breech plug wrench, some heat and penetrating oil. I would recommend taking it to a gunsmith in your area.
Thompson Center should be the company that you are looking for.