According to the Duets anthology DVD, Sinatra saw Sammy perform while he was still a nobody in the Army. He had an act with his father and uncle. Frank went to see him at the stage door.
Frank Sinatra was considered a "Baritone" with more or less a 2 Octave range from F to F. He sang in HIS keys! If you know nothing about music, you don't know what the hell I'm saying!