Gregg performs "I'm No Angel" as Sid Mcginnis makes perhaps his only mistake in his 31-year run on Dave's shows, starting the song a half step higher but correcting himself as soon as Will Lee and Gregg come in.
In the next segment, Gregg begins to chat with Dave but is interrupted by a "surprise" walk-on by Redd Foxx.
Redd Foxx was the last of several other "surprise" walk-ons throughout the show's hour: Christopher Reeve, Willard Scott, Susan Saint James, Peter Allen, and a joke walk-in by Biff Henderson.
Gregg had sat in with the band, and he was keenly aware of that night's "surprise walk-on" theme, so he was fully prepared for Foxx's spot.
I uploaded another Gregg Allman appearance on Letterman, which includes two music performances and one full interview, here: https://youtu.be/qukNGG3bXek