Proving once and for all that there's NOTHING that Can't be found on the internet, I come across this series of TV bumpers for "Cartoon Network's" [adult swim] that crosses 1960s style jingles with bizarre imagery (I'll figure out a way to "Soundcloud" them later)...
Now after watching all of them, here's the Million Dollar Question...Are the singers JAM/PAMS, Johnny Mann, or an In-house company? (I honestly don't know, but I think we can rule out everyone else.)
Interesting...they sound like Johnny Mann - They are still doing accapellas like that. But those are like really dry or no reverb-- There were some on Family Guy years ago as well. The girl with the glasses sing I've heard sung 'money music & fun or music in the morning - something like that
(btw on J Manns website one of the singers of note is Don Shelton still singing!)
They could be parodies of J Mann cuts; but they sure sound similar musically ( which is an awful thing to say if they were indeed J Mann's work, but you know what I mean) It seems the jingles J Mann is doing now are still top notch-- His singers may vary a bit is my guess too.
Have a look at this page- to the 8th video down; J Mann singers session clip- Ahh the things I find looking for things I didn't know about :)