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.  It’s been worked over and turned into a Frontier Sporting rifle. Some of the modifications include the addition of a dovetailed  U-notch  rear  sight ( poss. Freund Bros.) , a 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 before on some of the early Custer guns. If memory serves me this was done by a Post gunsmith where some of the 7th Cav. troops were stationed.  I beleive it was done to lighten the trigger pull. There were a few known Western gunsmith that did this kind of work to include J. P Gemmer- St. Louis , Freund Bros.- Colo. & Wy. , J. P Lower- Denver, Co., and Walter Cooper-Bozeman , M.T.  This rifle came out of Montana so I would lean toward Walter Cooper . A wonderful example of the Old West !