I'm guessing it's just a new sequence. But they did open a new plant in Olney, IL and the sources I've found on-line date that to 1962, so maybe "M" was 1962 in Little Rock and "N" was for 1962 bikes made in Olney. Some of the later badges mention Olney IL along with Little Rock, but the badges I've seen from this period only mention Little Rock. But I've only seen a few, so that's nothing definite.
In 1962, AMF (the American Machine and Foundry Company) employed 755 Illinois residents, 450 of whom worked at their bicycle factory in Olney, Illinois making their famous 'Roadmaster' line of bikes...
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