Definitely pictures would help, and so would the complete writing on the barrel and lock.
The Cumberland Gap was the entrance into Kentucky from Tennessee, North Carolina, and Virginia, so a Cumberland style rifle is most likely a Southern Mtn style, but that would only be a guess.
Pictures and more description please. Does it appear to be a factory model or a custom rifle? I am assuming since you said used, you mean of recent manufacture, but you could mean 200 years old used. Which is it?
ETA: I found pictures of one at the Historical Trekking website. ERA stands for Early Rustic Arms. I can't post the link, but do a search for it.