And if it's the & Co, then it's {{PD-UK-unknown}}, but we can probably ignore that. The last of those two to die is Cundall, so using his deathyear:
Public domainPublic domainfalsefalse
The author died in 1895, so this work is in the public domain in its country of origin and other countries and areas where the copyright term is the author's life plus 100 years or fewer.
https://creativecommons.org/publicdomain/mark/1.0/PDMCreative Commons Public Domain Mark 1.0falsefalse
Notes
↑The Digital Commonwealth gives 1859-1870; the (U.K.) National Portrait Gallery gives 1860-1865, however, the copy at the Boston Public Library has the back included, which, while heavily damaged, includes the address "10 Bedford Place". Per the Royal Academy, Cundall, Downes & Co. were there from 1863-64. Also, by 1866 they were known as Cundall & Fleming so that's something. [1] specifically dates it as an 1864 photo, which, given I had narrowed it to 1863-4 already, I'm happy to buy.
Captions
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{{Information |Description=Carte-de-visite mounted albumen photograph of author en:Wilkie Collins. Dimensions, including mount: 11 x 7 cm |Source=[https://www.digitalcommonwealth.org/search/commonwealth:5425mh490 Digital Commonwealth] (Massachusetts) / Boston Public Library |Date=1863-4<ref>The Digital Commonwealth gives 1859-1870; the (U.K.) National Portrait Gallery [https://www.npg.org.uk/collections/search/portrait/mw159448/Wilkie-Collins?LinkID=mp82238&role=art&rNo=4 gives 1860-186...
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