Another thought, different countries have different standards of perfection... and there is more than one European Naked Neck breed.
Transylvanian Naked Neck
The German Poultry Assoc. Recognise naked necks in both single comb and rose comb in the following colour varieties:- S.C. black, white blue-laced, red, buff, cuckoo, wild brown, silver pencilled, millefleur or jubilee.
The Dutch Poultry Standards recognise Black, Cuckoo and White colour varieties of Bantam naked necks.
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The French breed Cou Nu du Forez is a Naked Neck breed related to the Transylvanian Naked Neck. Has white skin instead of yellow. Lightly tinted eggs (180-200), dual purpose breed, red eras, single comb, whitish to yellowish legs, Cock : 3 to 3.5 kg ; hen : 2.3 to 2.8 kg, ... IIRC this breed is always white feathered... Google translated eyes as rosemary white... not sure what that is at all in our terms.
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The game breed Malagache From Malay though often cited as Naked Neck breed is open to some debate, they do loose Neck feathers the hotter the climate, but skin is different, true ones might and I am unsure on this, but might be caused by a different gene... this is one of those debate things, about person x having purer birds than person y which matters in the gamefowl world. So not sure what real research done... and how much is just people being jerks or confusing different gene combos like birds bred to have big pore skin and Na gene versus birds bred to have tight pore skin like we see in the utility European version. There could be multiple genes at work.
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Italian Naked Neck, or Collo nudo italiana, has white ears, yellow skin, yellow legs,
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There is no consensus across Europe where the Na gene birds originated from... Africa, local mutation, and when introduced, I have seen some very old cultures cited as origin.
Based on the above I have been able to glean from the net, some folks probably have lines that are more like one European Standard or breed than the USA standard. Sort of cool rosecombs are allowed in Germany.
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There are game fowl Indian lines with Na gene, Mexican and American game fowl too. They go under different names... again debates on these birds on the Na Aseels I just don’t know.
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There are African dual purpose breeds too. Village breeds.
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My bad Na is proper gene notation btw.