any guesses of age of photo much appreciated

[l8nda]

Their clothing is from the late 60s or earliest 70s, so 1873 seems completely reasonable to me. I'm not convinced that the corners of this card are machine rounded rather than square corners that were just worn down over time. Even so, there were advertisements for round-cornered cards in the UK from the very beginning on the 1870s, so I don't think a round corner rules out 1873. I find it incredibly difficult to believe this was taken after 1875, even if they were very rural.