Board Thread:Questions and Answers/@comment-1327106-20170426030842/@comment-5774380-20170427200903

Hmm, I see. To be honest I never paid much attention to this issue, but you're right, we've traditionally been using the AP style for possessive words ended in "s" and CMoS style for references; although the CMoS technically accepts both practices as correct, so I'd say to continue using AP style for these cases (I also find unnatural to see the extra "s" after a word already ended with it).