Lloyd, that Lang is a real beauty. Crudgington & Baker (Vol. 1) illustrate a Lang gun with the provisional patent No. 1785 of 1867, that was made as a central fire gun. I think yours might have started as a pin-fire from the shaping of the fences, but anything is possible; sometimes the signs of a conversion are devilishly hard to spot. Also, Lang could have made new barrels for it, if he converted it. In gunmaking in the 1860s, it is challenging to be definite about anything.

Lovely engraving, great condition. You are going to have a lot of fun with that one. Very unusual.