I will try and keep this on subject. I have not seen that designation in the many years of collecting license plates. I am no expert on these, especially the early years like yours. I have talked with Dave before on designations and he doesn't pay attention to them at all and may be of no help in identifying this plate. i will check some of my resources, but a couple of guesses might be:
METER MAINTENANCE (First parking meter was in 1935, so this might not work)
METER MAID (Doubtful that they used this and the plate looks like full size not Motorcycle plate)
MUNICIPAL MAINTENANCE * Street Sweepers - Road Crews etc.
