Welcome to ALR! This is definitely a Southern Mountain Rifle, or at least a rifle made by a gunsmith who learned the trade there, before moving elsewhere. The architecture, hand-forged iron trigger guard, and walnut fullstock, are all typical. Jerry Noble lists several Masons in his Notes On Southern Long Rifles, Vol 1 , but none of them have the initial "J". Do you know where this rifle came from, by any chance?