It was not uncommon back when for shooters who had loads in their guns that would not fire- usually damp powder, or clogged flash channels, or both-- to remove the OS card, and pour out the shot, so that if the gun was ever fired, all that would come out was the OP wad and Cushion wads- usually not a fatal wound if someone is hit with them. I have seen this is several old shotguns, of various vintage, from the 19th century to the mid 20th century. All I can suppose is that the shooter, not have the proper equipment to clear the barrel, decided to do this as a way to protect his family from injury until he figured out how, or save enough money, to clear the load. Then as time passed, he forgot about it, and it never got done. The gun gets sold- because its not being used anymore, and the family can't afford to have something sitting around that doesn't work. Or, it gets passed down as an heirloom, and the kids and grandkids have no idea the gun is loaded with powder and wads. If they did, they have no idea how to take them out, either.
The latter is how I have been introduced to several such guns. People find out I am " into Black Powder " guns, and ask me if I can clear a gun they inherited that they think is loaded. I even get referrals from other lawyers in the community where I live. I'm the "Gun Lawyer".
I use a ball puller jag( large wood screw type) to run into the wads, and pull them. Plain water dissolves the powder under neath it, and soap tends to help the process at that point. I have put my patch worm down the barrel to help dig through the compacted powder, and break it up so it will come out faster.
The nipple has to come out, and the clean out screw, if there is one, has to be removed for adequate cleaning too. I really want to get a good look at that flash channel to see what damage has been done there. Expect to have to replace the nipple, because of the abuse the original one has suffered. I would only keep an original nipple if the gun is beyond repair, and will become ONLY a heirloom wall-hanger.