Bb Circular Cornet by Isaac Fiske, 1855
Isaac Fiske is widely thought of as maker of the finest quality brass instruments in the US in the mid-19th century. He is best known today for his pushrod actuated rotary valves that he called "key pistons" which he patented in 1866 and applied to all sizes of his instruments. This circular Bb cornet was made about a decade before and is quite rare today, but at the time was a popular form. There are only three of these known to exist and a fourth example pitched in C. The original ambrotype below was taken in the 1850s and shows a musician holding a Fiske Bb circular cornet and a Graves 4 valve Bb bass in bell over the shoulder form but with a mouthpipe for playing the instrument upright.
Most Fiske instruments existing today are of German silver, including the example in the fourth photo from the collection of Steve Ward, making this all brass example even more rare. Notice, also, the design of the braces and finger ring of these other examples match the 1861 catalog image, which seems to be the standardized design. All these details lead me to believe that the brass example is the earliest and probably made in about 1855, when the unique American style circular cornets were growing in popularity. In other ways, they are of the same design: the bells appear to have been made on the same mandrel, although the earlier has a larger rim diamer (5" compared to 4 7/8") and wider garland. Also, the bore diameters are the same at .433". The mouthpipe shank and mouthpiece are reproductions. It had been badly restored many years ago, requiring much careful work to bring it back to its original appearance.
For more Fiske history see Robert E. Eliason's Early American Brass Makers.