Information on John Howe can be found in THE NEW YORK STATE FIREARMS TRADE, Vol. 2, by H J Swinney/Tom Rowe, on pages 810 through 812. He is listed in St. Johnsville, New York (are you sure your's is marked Johnsonville?), from 1859 through 1874, but probably also earlier and later. He is listed in numerous directories of the period, but census information in him was not found, for one reason or another, so biographical information is pretty much a mystery at this point. There are two side-by-side combination guns by John Howe, similar to your's, pictured in the book. Both are of high quality and in nice condition, but your's appears better than either of them. Both of those guns are stamped St. Johnsville.