The developer pages were down pretty much all day (for us located in Europe at least).. and the downloadable developer guide for 10.5.1 that I had on file has a no working version matrix. And, I promptly ran into an issue that required the versioning matrix. Guessing what could be correct solved my issue, but it shows the danger with online-only documentation.
And.. the latest guide you can download is actually for CUCM 10.0(1), not 10.5(1)
we are working to get the 10.5 doc download accessible.