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.