I'd bet a bunch of vocal groups from the early days of vinyl would fall into this group. They're probably mostly in Las Vegas and Branson these days.

As a personal memory, I played a few gigs with The Diamonds(most known for their hit 1957 record "Little Darlin'") back in the early 2000s. They were having a bit of a resurgence after a PBS series aired. At one point the original bass singer was back, but he wasn't long for this earth and somebody else was in his place by the second tour I did.