Thank you very much indeed for your responses. In particular, thank you to JP and Jodi who both identified the source of the image.
Highlight the space below to find out the answer:
The exact date is unknown. Research suggests this is authentic, and dates from c. 1870-1880.
I got the source right and STILL got the year wrong. How embarrassing lol
oh well, got the top end of the estimate at least!