From the pattern’s description:
“Clothing styles among the mountain men and fur trappers were fairly simple and utilitarian. As their cloth garments wore out, they were naturally replaced with items made from brain tanned buckskin, the material which was most readily available through trade with the Indians. Sketches and paintings from the period show a clear and consistent style, with long coats and trousers of Indian tanned leather being quite common. Although buckskin shirts were not as common as cloth shirts, they were a popular item and quite practical in the cold climates in which the mountain men earned their living.”
For additional ideas on design variations, we strongly recommend that you study paintings and drawings and visit museums and examine existing specimens. This will help make your garment unique and distinctly yours.