I don't show chickens and never will and you may be correct about the 10% at shows and from judges. I was only correcting your comment
the American SOP states you can have up to 10% it does NOT STATE A PERCENT IN THE SOP
You will also not find Marans in the current SOP they were admitted too late for inclusion. there are several thread that have the standard on BYC and you can also get it from the Marans club sites..... not sure which one is the one that got them admitted but theirs would be the GOLD standard for Marans. I can't post links from my phone. maybe Wynette or math ace could post those links for you.
One last thing. Though the BCM was admitted late for the APA SOP book and its not in there, doesn't mean the American SOP is separate. They are the same thing. I keep the SOP that I have inside my book. They approved the SOP and that is what the judges go off of. Also in the APA book under cutting for defects applies to the BC as well. These things are not listed on the BC SOP because it talks about the BC but we use it. If the BC SOP states 10% ( I will check) than that's what the APA has approved. It would also be in the book if they were to revise and reprint it.