Very neat rifle. I don't think it's a PA gun but where, no idea. Looks like a 'parts gun' too. The buttplate clearly looks like a fowler plate that was cut down a bit and reshaped somewhat and is likely why the toe flat is so narrow.
Has this been reconverted, maybe an old reconversion? Tough to tell with the photos but trying to zoom in, some of the exterior lock parts kind of look more like replacement parts as opposed to what I would expect to see with that lockplate. Could be wrong but there is definitely something funky going on with the fitment of the pan, or maybe just the camera angle.
Box lid looks to be original - that's extremely cool! How long is this barrel?