TIL about the lyrics being from Jon Hendricks. Thanks.
He also wrote the lyrics for their versions of "Birdland" and "Four Brothers", and maybe four of the tracks on Swing, and probably a bunch of other stuff I'm not thinking of.
Oh, there we go, the scat solo starting at 2:30 in "Ray's Rockhouse" is Jon.
He does get a credit for vocals and lyrics in Discogs, but they don't say which track:
https://www.discogs.com/master/97226-The-Manhattan-Transfer-...