Hey there --
Thanks for your post! This sort of input is very helpful. However, I think most of the characteristics you mentioned are a lot more reliable in **adult** animals, and more reliable for animals in the hand, than for the hatchling photos we have in front of us.
>>Morphologically, milks have a short, rounded snout. Corns have a longer, squared snout.
Hmmm. The OP claimed that corns have a pointy snout. Squared or pointy?

I don't think either nose looks especially pointy OR especially round here. OTOH, the distance between eye and nose-tip does seem to be a bit longer in the corn. Unfortunately, the appearance of such subtle differences can be significantly changed in pics with camera angles and lighting, so IMHO that isn't reliable in our situation.
>>Milks have a cylindrical body and a thick, poorly defined neck. Corns have a "bread loaf" shaped body (cross section), with a slender, defined neck (comparatively).
Do you really see much of a neck in the corn pic above?
And which of these bodies looks more loaf-y to you? Which looks more cylindrical? The pic on the LEFT is the milk, in this case.

>>Milks have a comparatively short, thick tail (typical of a fossorial species), while that of corns is long and slender (typical of a semi-arboreal species).
Ahhhh, here's another one I never thought of. Thanks for posting it!
OTOH -- can you tell where the tails start in these pics? Tail length might be a good indicator when you have a snake in hand so you can turn it over....but can you really tell here?

>>Milks:
>>- 7-8 supralabials, with the third and fourth entering the orbit of the eye.
Right. Unfortunately, I could only find 6 supralabials in the OP's snake. Where am I missing one? Is there perhaps one at the back that I'm not seeing?

>>- anal plate single
I think this one is gonna be the single most helpful, simple, and reliable indicator. Unfortunately, we'll have to wait for the pic!
Thanks again for your post. I do appreciate people who can offer good info without irrelevant drama!
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0.1 Peruvian rainbow boa (Amaru)
2.0 Brazilian rainbow boas (Arco, Olho)
0.3 Honduran milksnakes (Chicchan, Chanir, Hari)
1.0 Thayeri kingsnake (Coatl)
0.0.1 Mexican black kingsnake (TBA)
2.7 corns (Cetto, Tolosa, Uce, TBA)
1,000,000.1,000,000 other critters