There might be a reference that would give you the exact date in Calvet's
(translated) biography of Barthes.
 
But I heard a talk once by Norman Bryson in which, as I remember, he had
actually tracked down the cover and it turned out it was not really a
soldier but *a young boy* in uniform (this may be why researchers don't
find the cover that fits Barthes's description).  As I remember, Bryson
felt that Barthes's essential argument remained, but there had been some
interesting selective memory going on in Barthes's citation of the cover.
 
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