Hey! Cool!!! I'm looking at a brother to the gun I own. Mine is in a state of disrepair... the stock was full of dry rot when I got it, triggerguard bent and torn out of the mortice, well pitted percussion conversion, etc... BUT the engraving is identical on the triggerguard, similar on the buttplate, same lock bolt heads and washers, same architecture, etc. Yours appears to have been nicer quality from the start. With the lock reworked you might have a nice shooter if the bore is ok.
There are some things that jump out to me as not being right. I might be way off, I'm just telling what I see from the pictures, so please take this with a grain of salt. I think it's been restored at some point, the barrel cleaned up and the lock reconverted. The stock and brass bits look just right... they've got lots of bumps, scratches, and evidence of use (especially a broken forestock). Stock might have been refinished, the details at the tail of the lock panels are really worn down, but have nice finish. Likewise, I get the feeling the checkering might be newer too.
The engraving at the tail of the lock is good, what's left of it is pretty typical of the lower to mid grade English locks of the time. The engraving under the pan is newer, by a different and unpracticed hand. Also like you said, it could have never been fired with such a soft frizzen and geometry like that.
The barrel is what I realy don't get... it's beautiful. The front sight is really nice, no evidence of pitting anywhere up and down, and still looks like a tight fit in the stock. It appears to have the same sheen as the lock does. I dunno!??
Regardless of its past, It's a very nice gun with great lines and looks. The stock on mine is mahogany (yep!!), I can't tell what yours is. I'll take some photos of mine for comparison. Thanks for sharing!
-Eric
PS, does the buttstock seem heavy at all? Mine had 1lb of lead poured into it, but I don't know if it's originl or done afterwards.