Hi Copernicus...same problem, I'm afraid.....
However, this photo shows the corners of the cardboard mount, and the corners are rounded. This probably means it was taken post 1875 or so, as before then the corners tended to be square. Rounded corners made it easier to slip a photo in and out of the pockets in photo albums which were becoming popular in the mid 1870s, as photos became cheaply available...
Cheers,
China