As I said,my initial application was 40+ years ago so my memory might be faulty but I seem to recall being required to get confirmation of identity from either a physician,lawyer,clergyman or a policeman.I guess that could have changed by the time you made your first application.All of my subsequent applications have been pretty uncomplicated...as long as I submitted my previous passport (which I always did).
My first US passport required only Naturalization Certificate. (last year)