Last evening I watched the 1949 film noir TENSION, which is the first score for which Previn received screen credit. During a scene in a cocktail lounge, the "Fashion Show" tune is played on a jukebox. I had always thought Previn wrote this music for DESIGNING WOMAN (1957) during which it is featured prominently. After that it turned up as source music in other MGM films including NORTH BY NORTHWEST and THE PRIZE.
The only other source music I can think of that has been used this often is Kaper's GREEN DOLPHIN STREET theme.