Not sure, but I think the Model III had a 2MHz clock. The Model 4 had a 4MHz clock, which was why you could get a fifth voice out of the Orchestra90 when it was hooked up to a Model 4.

I really need to clear off some desk space and haul one of the Model IIIs out of the basement. And/or install one of the TRS-80 emulators someone posted the URL to on Elbows a while back. (Got 'em bookmarked.)