I was just checking out the mailbag archive and noticed that in response to the question “Is the guy on the Quaker Oats can John Penn?”, the answer did not include a note that America’s most famous Quaker (not counting Tricky Dick - whose ties are a bit suspect) was actually named William Penn. Did William have a brother named John who was supposed to have appeared on the can?
Hey, yeah, me wanna know, too! Who is “John Penn” and why is he in the Mailbag? 
Is the guy on the Quaker Oats box John Penn?
Nobody comes up on Google under “john penn quaker” except for the Mailbag article.
There were several members of the Penn family who were called John, including William’s son, John (1700-46), and his grandson, John (1729-95), lieutenant-governor of Pennsylvania, although the latter was never a Quaker. (A Google search actually does throw up some information about them.) One could imagine that there might be a specific portrait of one of those John Penns which was copied for the Quaker Oats artwork simply because it happened to match the common but misleading stereotype of an early-modern Quaker. That said, I think it’s just a generic depiction of a Quaker. I’m also fairly sure that it’s not based on any of the very few authentic portraits of William Penn.