It's 11:30 p.m. This has been on the Web 18 hours.
We should have called Strunk & White, or at least gotten off our asses to get our copy off the shelf, before invoking the two Cornellians. Indeed, The Elements of Style says to always use " 's " after a singular possessive proper noun.
However, the AP Stylebook, under the "singular proper names ending in S" section of "possessives," says this: "Use only an apostrophe. Achilles' heel, Agnes' book, Ceres' rites, etc.
This would seem to be the rare case where the stylebook and The Elements of Style disagree. We could have easily made the same mistake.