Wow...you've got a real winner there! From what I can see it looks like a "pattern" 1730/1740. Can you see the maker name and date on the lock plate? It's got that nice characteristically early banana shape although the cock itself is definitely a later replacement. Other than that and based on the photos, it appears to be 100% original and correct and retaining the original wood rammer which has got to be remarkable. Getting the cock replaced with a correct and properly-aged pattern is what I would do (your chances of finding an original are, well, likely zero) but there are different schools of thought on that. What all will agree on is that you should absolutely NOT touch it otherwise - no "refinishing", no polishing, no nuthin'! If you're interested, the best book currently on the market on the Brown Bess is called, interestingly enough, The Brown Bess by Erik Goldstein and Stuart Mowbray. I'm sure you'll find it through Amazon. I will not speculate on a specific value but having seen comparable untouched examples of somewhat later patterns on the market (and their actual sale prices), I'd say it's well into the five figure territory.