Where do you get a factory letter on it?....
Cody Firearms Museum/Buffalo Bill Historic Center is the caretaker for old Winchester records. Start here:
Neighbor freind took some whitetail with a similar vintage 38-40. BP level loads out of a long barrel are very polite and your ears can easily tell if you hit bambi or a tree. When you fall in love with shooting it consider that soft lead will easier on your vintage barrel than modern hard jackets.