Can't answer your qustions about this specific gun, but Col Clay Mosby was Newt's antagonist in the two series "Lonesone Dove: The Series" and "Lonesome Dove: The Outlaw Years" which ran in the mid-90s (94-95 or 95-96 if I remember correctly). These were regular season TV shows, not the mini-series like Lonesome Dove or Streets of Laredo.