Why are you calling one S?
D is Duplex, and R is Rose.
But in all the literature I can find, the other one is P for Pea.
S is Silver (sex linked silver/gold.)
http://kippenjungle.nl/sellers/page3.html
"Single" comb is the wild-type form, and is not-rose, not-duplex, and not-pea. So calling...
Why do you say that?
https://www.researchgate.net/figure/The-Duplex-comb-phenotypes-V-shaped-and-Buttercup-are-both-associated-with-a-novel-20-Kb_fig3_273780882
This page says they found a different bit of genetic code for one than for the other.
My understanding of the comb genetics matches yours. Definitely puzzling!
I'll tag some of the genetics experts, to see if any of them know what is going on here.
@nicalandia @ColtHandorf @Amer