Awesome find! I agree that it might not a mass produced gun model. I bought a musket a few months back for around $150 after seeing it on the floor of an antique shop, and never was able to find a matching one. It is complete, non-dug and with maker marks, it looks like it should be pretty IDable. Anyways, the barrel ended up being from around the mid 1700's or so based on the proof marks and shape, and then converted to percussion cap later on, but the mechanism is a percussion type one from the 1840's. Your look plate also is likely one of a kind, but I am pretty sure it dates from at least before 1840, when flintlocks were either being converted to take percussion caps or replaced by percussion muskets. Considering that most flintlocks still in use were converted to percussion by around 1850, I would feel safe saying that your gun was probably lost before then, and likely a bit earlier. Yours probably dates from the mid to late 1700's until the early 1800's