Hi road people, I've been lurking on this board for a while but got interested in this thread thanks to an email from Denny. Lot's of good stuff!
Anyway, I have a 1927 PP-OO guide so it does exist. Cover looks just like the 1926 but has fewer pages, few city maps. It shows US highways (Rand McNally was good about updating quickly). FYI, Anderson IN routing looks the same as in 1926. Rand 1928 IL map still labels PP-OO route, BTW, so they didn't drop the names immediately.
Also have a 1924 National Highways Assoc PP-OO map wich shows the route thru Lebanon-Crawfordsville-Danville-Chambana to corroborate an earlier post. I'm almost sure I've seen some map with both routes marked (the northerly thru Danville, Il and the older southerly thru Danville, IN-Crisman-Tuscola). The older route enters Decatur further south than the 1924 version, and that's where the existing "Ocean Trail" street is that I put a photo of on my website ppoo.org. Someday I'll get around to updating that site...