Does anyone know for sure when The South Bend Watch Company started to use painted metal dials on thier watches instead of porcelain?

It's hard to say "for sure" but there is evidence that the SBWCo started using metal dials on 12s watches as early as 1914.

For the 16s watches the earliest evidence comes from 1922. The metal dials for the 16s appear to have been first used around the time that luminous dials first became popular. They may have started in limited production 2 or 3 years earlier, so perhaps around 1920. I don't believe that metal dials were ever made originally for the 0s, 6s or 18s watches although the SBWCo may have produced replacement metal dials for the 18s watches in the late 1920's.