Does the diploma say Extension School?

Yes, in Latin. The diploma reads UNIVERSITAS HARVARDIANA; then, after the graduate’s name comes Magistri in Artibus Liberalibus Studiorum Prolatorum (Master of Liberal Arts in Extension Studies).

The diploma reads UNIVERSITAS HARVARDIANA; then, after the graduate’s name comes Magistri in Artibus Liberalibus Studiorum Prolatorum (Master of Liberal Arts in Extension Studies).