I heard back from the gentleman who currates the firearms for the Montana Museum of History. Here are some pictures he was gracious enough to send, along with what he knew of the rifles history.
"Here is a photo of the tang and breech from above. The breech in front of the snail is about 1” flat-to-flat; the muzzle is .875” (7/8) flat-to flat.
I hope this is enough to get you going. Please let me know from time to time how you are progressing. If you get stuck, don’t hesitate to contact me.
This really is an unusual firearm. It was purchased by a Kalispell, Montana collector sometime in the first third of the 20th century at Webster Grove, Missouri. Webster Grove is a suburb of St. Louis and coincidentally was the home of Jacob Hawken’s son, Christopher Hawken. His house still stands, although it has been moved from its original location.
I wish I knew more about this rifle."