When I say "interpolated", I doesn't mean that they were inserted later--it means that at the time they were produced, existing songs and other performance pieces were put into the play as a vehicle for certain performers--this was, is and has been a common practice in the theater--

In Shakespeare, think of the many instances where the main character encounters a fool and the story line basically stops while the character sets him up for witty rejoinders--

It is likely that these routines were expanded, condensed, or even replaced from performance to performance--it is also likely that well known performers interpolated the routines that they were best known for into a whatever play they happened to be in, in the fashion of Abbott and Costello's famous "Who's on First" routine--