The rifle in the original post was the subject of a full-page feature in Muzzleloader magazine a few years ago (apparently in the May/June 2015 issue). The web link provided by Elnathan (above) has a good deal more information than the magazine provided. The rifle was owned by a Mr. Jeff Hengesbaugh, who did the research and provided the information about the rifle. My take on his article is that he made a strong case for prior ownership by James Beckwourth. The Hawken attribution cannot be proven beyond a reasonable doubt, and may be less important or significant than the Beckwourth connection. I believe Mr. Hengesbaugh also believed the rifle was originally a flintlock, but I have my doubts on that.
Publication of the photos in the magazine prompted some discussion of this rifle on a couple of online forums. I think Mr. Hengesbaugh actually brought the rifle to Bent's Fort, for one of the events there, and let people handle and examine it. I was not among them, but comments made by others indicate this is a very heavy piece, weighing in excess of twelve pounds. Ruxton frequently used the phrase "long, heavy rifle" in his descriptions of the weapons carried by the mountain men of his acquaintance. Mr. Hengesbaugh indicated the barrel is 40-5/8" long, and tapered.
I appreciated Hlbly's comments and pictures. He makes a strong case for a non-Hawken provenance, and his rifle does indeed appear to be a near duplicate of the Beckwourth gun.
I corresponded with Mr. Hengesbaugh a little about this rifle back in 2016. He was planning to publish a book about the gun and his efforts to confirm the Beckwourth connection, but I don't know if that ever came to fruition.
Thanks for this interesting discussion!
Best regards,
Notchy Bob