We might be able to narrow this down quite specifically.

The man at the back on the left appears to be wearing the M1872 Officers' fatigue blouse, as might the man next to him. This was replaced in 1874 by the M1874 fatigue blouse, as apparently it proved unpopular.

Tintypes were at the peak of their popularity in the 1860s and 70s, although as photography advanced, by the 1870s they were being relegated more to tourist portrait shots and the like. That might be why this shot looks to be at Niagara Falls. Of course, if this a tourist shot, it opens up the possibility that this is a veterans' reunion and could be taken a while after the 1874 switch, which would also explain the civilian suits.