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Section “Necessity of naturality” sounded like there is nothing interesting going on if we look at the unnatural isomorphism of hom-sets. But this isn’t the case, already in the 60s a graph-theoretical and purely categorical facts of this form have been established. I’m adding the two earliest references.
Tomas Jakl
The entry fell short of really citing the following in full, so I have added it now:
Yoshiki Kinoshita, Nobuo Yoneda (1930-1996)
&
Saunders MacLane, The Yoneda Lemma,
Math. Japonica, 47, No. 1 (1998) 155-156
(pdf, also: posting to catlist in 1996)