My 2009 edition of the Standard Catalog of Firearms lists a Cooper, J. M. & Co.
A .31 caliber percussion double-action revolver with 4", 5",or 6" octagonal barrel, and a 6-shot unfluted cylinder. Blued with walnut grips. During the first two years of production they were made in Pittsburgh, Pennsylvania, and were so marked. Aproximately 15,00 were manufactured between 1864 and 1869.
Good $1650 Fair $675 Poor $200
Note: Pittsburgh-marked models add 20 percent."
I hope this helps you out.