That long top tang is a pinfire-era feature as well, but I see nothing else here to clue me in to any modification of the gun. The standing breech is noticeably pitted from digesting lots of black powder shells (& their mercuric primers), as was the the first foot of the barrel walls. If I'm missing something here, please... do let me know.
Do those nipples (and their placement) provide any clue? That design also looks to be a later type than what I've seen on other early guns (certainly different from the ones on center-fire gun sn 3359, and illustrated on page 126 of C&Bs book, included above), and... I'm fairly sure the hammers are replacements as well (the screws look like they came from a hardware store...I may need to deal with that).
The old case fits pretty good (it certainly pays to have friends in the fine gun-world), eh? It even came with a period correct(?) cleaning jag. I need to clean the case up a bit and do a few repairs, and I even have a period-correct (& address-correct) Joseph Lang & Sons label for it now. This old girl is going to be lots of fun and...I won't have much tied-up in it (which also satisfies my ever-frugal Scottish-nature).
I just checked and Kirby still has this gun up on his webpage. I looked at this unit for well-over a year before finally buying it (from our conversation, I suspect he'd had several refusals/returns on it before my purchase of it, as the price was steadily going down that whole time). His photography is really, very complete and is quite good (and can be blown-up on his webpage very nicely. I stared at it [& many others there] for a very-long time before doing the deal). I'd think that for most folks, this gun is simply too-unconventional (bar-actions & top-levers are far-more comfortable investments), but... for me (and my admittedly-eccentric tastes), this one was exactly what I was looking for. TIme will tell, of-course.