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Scaleless mutant in chickens is a single recessive gene. The name is from lack of scales on legs and feet- look closely at his pictures and you can see the legs and toes are completely smooth. What it does is interrupt follicle formation in the skin, thus preventing feathers, scales from forming at all, leaving the skin very smooth. Some or many of them do have occasional and completely random feathers growing on their body, these are the areas where the processes in the skin still manage to get some chemical action going enough to produce a follicle. Interestingly, they can be selected and bred to go either way- for much more naked, less feathers or for more feathers.
As to it being new/old mutant, in the Crawford book of poultry genetics, it has a 'discovery date' of 1957 in a New Hampshire flock reported by Abbott and Amundson. It's very likely it has popped up before- it still shows up 'randomly' in various flocks, I've seen pictures of a completely naked Oriental type rooster in an Asian country, can't remember which one it is at the moment and on an Australian forum, one showed up in a hatch from a backyard flock and is currently being raised(this one's a pullet and has a few random feathers). Both of them have completely smooth legs so it's likely the same scaleless mutation.
No idea if it's analogous to scaleless in snakes. My guess is 'probably'...