I seem to remember hearing that he was brought in to do the vocals for “Letter” by the Box Tops, because their regular singer wasn’t able to be there for the recording.
It’s possible.....
“A local band recruited the teenager in 1966 as their lead singer after learning of the popularity of his vocal performance at a talent show at Memphis’ Central High School; this band was The Devilles, later renamed Box Tops.”
http://en.wikipedia.org/wiki/Alex_Chilton