Easy AU on technical merit. Me, I want to see luster on Walkers for high AU grades but in strict technical terms it's no worse than 55.
It's the "micro" mint mark. The larger MM is bigger than this to the naked eye. It doesn't nearly fit into the "notch" as this one does, being (to the eye) nearly twice the size. San Francisco was transitioning to larger MM's in this period for readability. I do not think it's "listed," but it maybe should be. NGC identifies a 1942-S in that fashion. Notate it and hold for the long-term, against possible future recognition of the type.