Stevie Wonder is somewhere between jazz and popular music. The AutoValve harp is a diatonic harp, tuned like a standard diatonic, except that it is two harmonicas, one above and one below, mounted in the same body. The lower harmonica gives the sound warmth and depth, while the upper one gives it brightness and snap. I wrote an article about it in HIP No. 2 and demonstrated it on a companion cassette. One reed is slightly detuned, giving the sound a certain warmth and body.
Accordions also have tremolo tuning, and the sound is similar.
Gordon Jackson, who is on this list, is something of an expert on tremolo instruments, which are primarily used in folk music of Europe and Asia. They don't seem to have much currency in North America, except in French Canada. Most tremolo harmonicas are diatonic. Some models come with several keys mounted on a spindle like a corncob - rotate it and eat another row in a different key. If a harmonica has Echo, or an alpine scene on the box, it's probably some kind of tremolo.