It is actually a topic already discussed on the Cafe, if you search on it I'm sure you can find it. or...... Here is the short version
M = kempo CHINESE N = kenpo Japanese
Whether the art is spelled with an N or an M is the same system. The letter N is a representation of the Japanese Interpretation of the Kenpo system, and the Chinese Interpretation is spelled with the M or Kempo.
Sijo Emperado, and the late GGM Aleju Reyes signed a document giving. GGM. Reyes first 2 Black belts, GM Don Nahoolewa and the Late GM Richard Peralta the right to use the spelling with an M - KAJUKEMBO. Any schools that use the "M" can trace their lineage to these two men.