They are probably just from different periods in Noritake's history. They should help you narrow down the date of production--check here:

Here is a link that might be useful: http://gotheborg.com/marks/noritake.shtml

I can tell you this they are old the "M" means morimura here is alittle info

Nippon with the "M" dates to 1914

just Nippon no "M" dates 1904-1914

Morimura dates further back cause they came out first then Nippon stepped in. noritake was after

here is a site that might help you it explains the difference

http://www.noritake.co.jp/eng/company/morimura/history.html

Here'as more info.
http://www.gotheborg.com/marks/noritake.shtml