Old "Homage to Catalonia" cover

I have an old (1969) Penguin edition of H. to C. The cover is credited with
the words "Painting by Patrick Procktor". It's in fact a watercolour showing
a group of militiamen giving the antifascist salute from a railway carriage,
an obvious reference to a passage from the book (in which it's said to be
Italian militiamen).

But, some years back I happened to find the photo from which this painting
was lifted. I saw it in a Spanish photo magazine, back in the 80's, where it
was stated that those pictures were being published for the first time. The
painting is just a copy of the photo. But I saw that years ago, I don't
remember the name of the photographer and that magazine will have been
dumped (sorry, "recycled") by my wife by now. Any help?

Incidentally, a Google search lets me know that Patrick Procktor died last
month.

Regards,
A.P.
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