Given that Robert Crisp's Stuart was penetrated by German tank guns six times while fighting Pzkw IV, Pzkw III, and Pzkw II tanks, the hits must have been from Pzkw III's firing 50mm KwK 38 L42 guns. Their penetration, while firing APBC shot was:
This is from a page "German AFV in Normandy". I don't know accurate they are, but they seem plausible. Now, we will look at Stuart armour (from OnWar.com. I the thickest armour on the front was 38mm, but most was 25mm that could be penetrated out to 1500 meters. The 38mm could be inside of 500 meters, if the angle were not to acute.