This is a really neat and scarce  Western Frontier  conversion of a mdl. 1873 trapdoor rifle.  It’s serial # 15202 puts it’s original date of manufacture at 1874. The 24″ barrel is .45-70 cal.  It’s been worked over and turned into a Frontier Sporting rifle. There were a few well known Western gunsmith who did this kind of work to include J.P. Gemmer-St. Louis, Freund Bros. Colorado & Wyoming, J.P. Lower , Denver, Co., & Walker Cooper, Bozeman, Montana. This rifle came out of Montana Some of the modifications include the addition of a dovetailed  U-notch  rear  sight ( poss. Freund Bros.) , a antique U.S silver coin is used for the front sight. Also, the sports style stock has a pewter forend cap added  and a steel buttplate. A rib under the barrel with two pipes for a wiping rod were also added.  I also see where a screw was added to the lock above the trigger . I’ve seen this  modification before on some of the Custer trapdoors . If memory serves me this was done by a Post gunsmith where some of the 7th Cav. troops were stationed.  I believe it was done to lighten the trigger pull.    A wonderful example of the Old West !