I am not sure what it is, however I am looking at the forearm which does not appear to have a metal cap and also the barrel band. I am no lever gun expert at all but the forearm treatment may contain some clues as to the rifles identity.
In addition, there are no obvious indications of "photo-shopping", however in the age the photo was taken or indicated to be taken, photographic film was very slow requiring long exposure times. How did they get the horse with the man standing on his back, to stand still that long time? Just food for thought.
