I don't know if this will help but although I can tell all our chickens apart ( even the ones of the same breed) we have two RIR's that have looked strikingly different from each other almost right from the start and not just feathering but also body structure and shape. There were several weeks where we suspected that one or the other was a rooster. Someone who knows chickens better than I do suggested that it could be a matter of interbreeding with another breed many generations back.
We have Puli's which are a very rare breed of dog and though we have "papers" on her going back five generations, one of ours is very far breed off breed standards. Her legs are long, her coat is wiry and will not cord, most of all she barks incessantly and we suspect she would chase a ball over and over until she dropped dead of exhaustion. While researching we learned that about 200 years ago there was purposeful cross breeding with french terriers to increase prey drive so I suspect that this is just as possible with chickens.
Just a though.