If you ever get a chance to drive through Pennsylvania, you'll find a lot of things named for keystones. There's a Keystone state park and a Keystone state motel, for instance. There's also a popular beer called Keystone. Personally, I think the theory that PA is the central keystone of an Original Colonies arch is the right one. John Morton casting the deciding vote for American independence was an important moment, obviously, but I think the nickname was already in the works before the Revolutionary War.