Hello. Welcome to the Forum.
Your gun is certainly a head-scratcher. LOL Here are a couple things I notice:
The LOP (Length of pull) - what we normally define as the length between the trigger (in this case the front of the brass lever) and the rear of the butt stock is only about 8-8.5". The only historic guns I know of that have a rifle "shape" butt stock and a LOP that short, are 19th Century Ottoman/Turkish blunderbuss. Collectors of Middle East type guns often nick-name them Ottoman Knee Pistols. While the butt is shaped like a shoulder gun, they were actually managed as pistols. So, I think that is what the gun stock started out with originally. The fore end of the stock cut back and the original blunderbuss barrel replaced with the current barrel. The silver decoration on top of the barrel looks some-what Ottoman, but a bit different. Hmmmm....can't put my finger on it at the moment. The lock appears to be a late period flintlock with a roller on the frizzen spring. Likely of European origin. The lock looks like it might be a period replacement due to the less than desired fitting in it's lock mortise. (?) It does indeed look like it was re-made for the trigger guard (which also looks Ottoman) to press up on the brass lever to fire the gun. Strange to go about all this extra trigger work unless there was a reason. Funny, but the comment above suggesting it might have been done for someone short on fingers actually makes sense. From the photos, it appears the entire gun was re-made during the period. But as mentioned above, it would take some dis-assembly to see if period done or a compilation of antique parts assembled by someone in the 20th Century for display. But at the moment, I'm betting on the first scenario.
Speaking of dis-assembly: I notice you live in PA. There are a ton of muzzle loading gun enthusiasts in your State. Likewise, there are likely many black powder clubs. Probably one within reasonable distance from where you live. One of the guys could dis-assemble the gun further to study. The primary goal is to see if the gun appears to have been made this way back in the period, or assembled later by some hobbyist. In this case, it would be worth knowing. Especially with the odd trigger/lever assembly.
Rick