3rd model Colt Dragoon
FWIW according to the 1999 catalog of Colt Blackpowder Arms the 3rd. Model Dragoon was available in 3 variations at $ 499,95 each: with fluted cylinder #F1743, with steel backstrap, cut for shoulder stock #F1741, with brass backstrap #F1740. The supportive text is interesting. It reads "One of the most unusual Colts is the Third Model Dragoon with fluted cylinder. Only 12 units were produced, on an experimental basis, in an attempt to reduce the bulk and weight of these large revolvers. Sam Colt abandoned the idea, and the experiment ultimately led to the 1860 Army design."